A constant accusation against Catholicism is that the Roman Church places mediators between men and God in addition to that of Christ - our one mediator (1 Tim 2:5). But a closer examination of the passage shows us that St. Paul commands us to intercede for each other:
"I urge that supplications, prayers, intercessions, and thanksgivings should be made for everyone, for kings and all who are in high positions, so that we may lead a quiet and peaceable life in all godliness and dignity. This is right and is acceptable in the sight of God our Saviour, who desires everyone to be saved and to come to the knowledge of the truth. For there is one God; there is also one mediator between God and humankind,Christ Jesus, himself human, who gave himself a ransom for all—this was attested at the right time." -1 Timothy 2 (NRSV)
Catholicism confuses alot of people (myself included) because of it's complexity, 2000 years of Tradition filled with monks, councils, and canon law make almost every issue very full of discussion and various decisions. There are two sides at least to every issue. In terms of mediation, Catholicism actually dogmatically requires no intermediary for salvation but Jesus Christ. The Council of Trent states that the sacramental salvic graces of baptism and absolution can be granted by God merely at their desire. As well, while Mortal Sin requires priestly confession for absolution by the same council, the Catechism of the Catholic Church also states that Mortal Sin can be absolved by the penitent's perfect contrition (sorrow for sins because the penitent loves God and has offended Him).
Therefore dogmatically, a person without any priest or church can - according to Catholicism - be saved by Christ. But, the thing that confuses people is that this is not the normative mode the Church believes to occur. Rather, God has ordained the Church to be his vessel/vehicle of salvation. He has left us a priesthood to offer the Eucharist, and to offer pardon for sins in his name. The problem with mediators is that people see them as obstructions rather than bridges or highways for God's grace.
Personally I get worried every time I go to confession - worrying I won't be forgiven. But each time I am absolved, I am reminded in a real tangible way, that God has forgiven me.
So again the question remains, why have a mediator? It's a question I've asked alot myself, and I've found the answer to be, appropriately enough: because God desires it. I've learned that God is understood in Catholicism as wanting to incorporate people into his work, he has appointed angels to carry prayers (see the book of Daniel), apostles to spread his word, the church to administer his sacraments, and saints (living and dead) to offer intercessions.
Someone explained to me at the beginning of my foray into Catholicism that the saints should be signs pointing to God, and God should not be viewed in the pagan sense, as competing with his creatures for glory. That he freely gives his work to angels and men to carry out, and to share in his eternal glory.
The best explanation of the Catholic understanding of God is actually in J.R.R Tolkien's "The Silmarillion" in the creation story, where the one God creates angels/demi-gods to create and watch over his creation, each is given a task and they work together in harmony. Men end up worshipping these demi-gods as polytheists accidentally. Thus Tolkien also explains his theory of how polytheism and Natural Theology / Common Grace fit together. Just another unlikely way that Tolkien helped me understand Catholicism.