Some of them make a positive contribution and some don't.
I find Francona to be excellent.
He is like a "with it" father to the players, and from the games I'm seen him manage (yes small sample size) he has seemed to be ready/has a plan for every twist. Even if strategy-wise he's just doing as he's told, he is still good.
Then you get guys like Dusty Baker who are stuck in the 1950s style of managing. They probably cost their teams a handful of wins a year with favouritism and "hunches".
If someone tells you managers have no impact, or an enormous impact they are both wrong.