Bill,
You have some misconceptions of the Catholic belief about Mary. The Immaculate Conception is a doctrine about how Mary could be part of humanity redeemed by Christ, but also be completely pure and without sin. It is not a matter so much of a benefit needed for Jesus, but of Mary’s role as the first member and mother of the Church. There are many parallels between Mary as the New Eve and the Church as the bride of Christ. Eve was created without sin, but said no to the plan of God, which led to the fall. Mary was created without sin, and said yes to the plan of God, which led to the new humanity. The Church is indwelt by the Holy Spirit and gives the Word of God to the world. Mary was overshadowed by the Holy Spirit, and brought forth the Word of God to the world. The doctrines about Mary are so rich, I can only begin to scratch the surface.

Here are some references that may help you out with a better understanding of these doctrines.
First, if you’re willing to do some lengthy reading, I recommend The Dogmatic Constitution on the Church [ http://www.vatican.va/archive/hist_councils/ii_vatican_council/documents/vat-ii_const_19641121_lumen-gentium_en.html ], which is one of the two foundational documents from the Second Vatican Council. Chapter 8 (starting in paragraph 52) focuses on Mary and her role and identification with the Church.
A much shorter, less in depth, and a little less authoritative source is from the Catechism of the Catholic Church about the Virgin Birth of Jesus [ http://www.vatican.va/archive/ENG0015/__P1K.HTM ], which also gets into doctrines about his mother.
Finally, I’ve skimmed this article from the Catholic Encyclopedia [ http://www.newadvent.org/cathen/07674d.htm ] which looks pretty good. It’s not authoritative, but they usually do a good job and provide lots of background material.