Michael Cohen was Trumps attorney. He did not have to say anything about Trump at all. Lawyer client privilege and all that. Mueller found some illegal dealings on Cohen and used that to leverage testimony from Cohen on Trump. The things they got on Cohen were not on Muellers to do list as "special prosecutor".

Liberals are like a "Slinky". Totally useless, but somehow ya can't help but smile when you see one tumble down a flight of stairs!