Here are Catholic websites that answer the question: “Why do Catholics pray to Mary?”
Each one of the sources below gives the same answer: We pray for her intercession.
http://www.catholicdoors.com/faq/qu67.htm
http://www.patheos.com/blogs/standingonmyhead/2012/04/do-catholics-worship-mary.html
http://www.catholicscomehome.org/mary-the-saints/
Look around you.