question about Hitler and the S.A.?

I don't understand why Hitler made the S.A. really big and even revived it after the failed overthrow of the government with ERnst Roehm. Then he purged the S.A. and its leader Weren't Ernst Roehm and Hitler great friends as well? So why did he revive and then destroy the S.A. and his friends? if somebody could help clear some things up itd be great :3