The word “Wasilah” (means) has a wide range of meanings which include anything or any action that helps get nearer to Almighty God. In view of the fact that the universe has been created on the basis of the intricate system of cause and effect, causes and caused for guidance and perfection of mankind, and just as man’s natural needs are met through material causes, in the same way the divine blessings such as guidance, forgiveness, nearness to God and man’s promotion to higher spiritual positions are based on a particular system and that they are realized through specific causes and means without which it is impossible for man to get those blessings and to attain proximity to God. These means, without which attaining proximity is impossible, have been introduced in the Quranic verses and hadiths.
To answer this question we must first shed some light on the meaning of mediator or means:
When it comes to the meaning of ‘means’ in the holy verse:
“O you who believe! Fear Allah and seek means of nearness to Him” 
Allamah Tabatabai explains the term as such:
“The reality of the means of nearness to Allah is to comply with demands of His path with knowledge and worship, pursuit of virtues and fulfillment of recommended acts. It is a sort of joining together, i.e. a spiritual joining which connects the slave with his master; there is no link that connects man with his Lord except the submissiveness of worship and veneration. Therefore, the means of nearness means ensuring the reality of veneration and turning the face of humility and poverty to His direction – therefore, Wasilah (means) in the above verse refers to the link that joins the servant to his Lord.” 
Wasilah has a wide range of meanings; it includes any action or anything that brings us nearer to Allah, the most important of which are belief in God and his Prophet, Jihad and acts of worship such as prayers, Zakat (alms), fasting, (Hajj) pilgrimage of the House of God, visiting the relatives, giving alms, openly or secretly, and any good acts…and also the intercession of prophets, Imams and pious servants of God which helps an individual gain nearness to God and which, in its broad sense, is included in Tawassol (invocation). Also, following the Prophet (pbuh) and Imams and walking in their footsteps bring us nearer to our God. Even imploring Allah to swear on the position of the Prophets, Imams and Allah’s saints has such a virtue…” 
In a commentary on the aforementioned verse, it has been stated in Tafsir Nomuneh that: Wasilah has a wide range of meanings; it includes any action or anything that brings us nearer to Allah, the most important of which are belief in God and his Prophet, Jihad and acts of worship such as prayers, Zakat (alms), fasting, (Hajj) pilgrimage of the House of God, visiting the relatives, giving alms, openly or secretly, and any good acts…and also the intercession of prophets, Imams and pious servants of God which helps an individual gain nearness to God and which, in its broad sense, is included in Tawassol (invocation). Also, following the Prophet (pbuh) and Imams and walking in their footsteps bring us nearer to our God. Even imploring Allah to swear on the position of the Prophets, Imams and Allah’s saints has such a virtue…” 
Therefore according to the Quran, which has ordered the believers to seek means of nearness to God, Wasilah or imploration has one such meaning.
In addition to the aforementioned verse, the Holy Quran says in verse 97 of Surah Yosuf:
Josef’s brothers requested their father (Jacob) to seek divine forgiveness for them and their father fulfilled their request.
Ibrahim’s act of seeking divine forgiveness for his father has also been mentioned in Surah Tawbah of the Quran which indicates the role of Prophets’ prayers in the forgiveness of peoples’ sins. 
In many narrations passed to us by Shia and Sunni reporters, we can see the role and significance of imploring Allah’s’ saints for forgiveness.
It is mentioned in Vafa ul-Vafa of Samhudi (a Sunni scholar) that it is permissible to seek the holy Prophet (pbuh)’s intercession and help before and after his birth and also before and after his demise or in the intermediate world and on the Day of Judgment.  Thereupon, he has quoted Umar bin Khattab to have said that Adam sought Prophet Muhammad’s mediation, “Adam knew in advance about the creation of Muhammad (pbuh) so he appealed to Almighty God by saying: “O’ Lord, forgive me for Mohammad’s sake” .
Also, he relates another hadith from a number of Sunni narrators including Tirmizi and Nesai who said: “A blind man asked the Holy Prophet (pbuh) for a supplication to cure his disease; the Prophet (pbuh) ordered him to pray as such:
“O’ Lord, I ask you for the sake of your Messenger, the Messenger of mercy and forgiveness, oh Mohammad I am turning to you in the presence of God to fulfill my wishes and O’ God, make Mohammad my intercessor.” 
Bayhaqi is quoted as having said that during the time of the Second Caliph there was famine for two years in the Arabian Peninsula. Bilal, accompanied by some other companions (sahabah), went to the Holy Prophet’s grave and said: “O’ Prophet Seek rain for your Ummah because they are about to be destroyed .” 
The question to deal with is: Why do we need a means and a mediator? The answer is that the universe has been created on the basis of cause and effect for man’s guidance, growth and perfection. His needs are fulfilled through material and sometimes non-material causes and means. In fact, mediators or agencies serve as causes or means through whom proximity to Almighty God is gained. Divine blessings such as guidance, forgiveness etc. are sent down upon man by way of a specific order. God is Wise and He has willed that such matters take place through special causes and means. Therefore, it is meaningless to ask why in the material world the Almighty God has created the sun to brighten the earth. Why God has made water a means for quenching our thirst? And why doesn’t God fulfill these needs directly without these means? In the abstract world also, we cannot make such questions as to why Allah (swt) doesn’t bestow His mercy, guidance and proximity upon man without a means and a mediator. Thus, just as a plant cannot grow up without water, soil and light, man is also unable to reach the ultimate blessing without any divine means.
Martyr Mutahhari says:
God’ actions are based on a system and if someone is heedless of it, he is astray and deviated. For this reason, God, the Exalted, has ordered sinners to go to the Prophet’s house and should, in addition to seeking divine forgiveness on their own, implore the Prophet (pbuh) to pray to God to bestow His Mercy upon them. 
The Holy Quran says:
“If they (unbelievers) ever do injustice to themselves and come to you (Muhammad) asking for God’s forgiveness and if the Messenger also was to ask God to forgive them, they would certainly find God All-forgiving and All-merciful. .” 
It is because of fulfilling this divine tradition that so much emphasis has been put in Quranic verses and narrations on seeking means for forgiveness and nearness to God.
Almighty God says:
O you who have believed, fear Allah and seek the means [of nearness] to Him and strive in His cause that you may succeed. 
In his discussion about intercession, Allamah Tabatabai says about the role of intercessor as such:
In reality the interceder adds his own recommendation to the plea of the petitioner; in this way the number of pleaders becomes even, and the weak plea of the petitioner is strengthened by the prestige of the intercessor. Had he used his own power, he would not have been able to achieve his goal because his power was weak and insufficient. Intercession is a means which supplements the cause. 
It is necessary to make mention of the following points in the closing:
1. Although man’s materialistic needs are met by worldly means, it doesn’t mean that non-material means such as prayers and imploration for help do not play a role in fulfilling our material needs. Divine means are sometimes cause-makers and at times cause-destroyers. That is, sometimes it is the material causes that have an effect but the grounds for achieving these causes are paved by means of supplication and prayers and at times the supplications eliminate the effects of the material causes and make them devoid of any impact. For example, a flaming fire that should burn something does not burn. Anyway, the cause of all causes is Allah and He can put in the spiritual means the same effect as that of a medicine.
2. The concept of intercession the Prophet or Imams or Allah’s saints is to make recourse to the high position of these great people because they are dear and near to God and if their intercession is sought, God will, for their sake, grant His servants the position of proximity. 
 Al-Maidah: 35
يا ايها الذين آمنوا اتقوا الله و ابتغوا اليه الوسيلة
 Allamah Tabatabai, Al-Mizan, Vol.5 p.535
 Makarem Shirazi, Tafsir-e Nomouneh, vol.4 pp.364-367
 al-Tawbah: 114
 Samhudi, Vafa ul-Vafa, vol.3, p.1371
اللهم اني اسئلک و اتوجه اليک بنبيک محمد نبي الرحمة، يا محمد اني توجهت بک الي ربي في حاجتي لتقضي لي اللهم شفعه في
Vafa-ul Vafa, vol.2, 1372 (ibid).
 يا رسول الله استسق لامتک فانهم قدهلکوا …
al-Tawassol ela haqiqat al-tawassol, p. 329, see Vafa-ul Vafa, vol.2 pp.368 – 369 .
 Tafsir Nomouneh, vol.4 , pp. 369 – 368
 ولو انهم اذظلموا انفسهم جاؤوک فاستغفروا الله و استغفر لهم الرسول لوجدو الله تواباً رحيماً
 al-Nesa: 64
 Collection of Shahid Mutahhari’s Works, Vol.1, p. 264
 Al-Maidah: 35
 Translation of Al-Mizan, Vol.1, pp.239 and 240.
 See translation of Al-Mizan, Vol.1 pp.239 and 240; al-Mizan, Vol.1, pp.239 -246