Zac Muad'Dib's Question

Alright, so I have read Dune now. I have, I believe, come to understand most of the plot and sub-plots pretty well, however, I cannot understand the Bene Gesserit thing yet. I understand that the Bene Gesserit practice Controlled Genetic Breeding so that they might create the Kwisatz Haderach, which is the name for the prophesized messiah, yet, I do not understand the purpose of the Bene Gesserit doing this, or why the Bene Gesserit look down on Paul and Jessica after Paul turns out to be the Kwisatz Haderach.