Managers boss people around tell them what to do, cause the people are too stupid to know for themselves what to do.
In other situations things would be chaotic with out someone organizing things and being the final voice, teams wouldn't know to bench who or to let who play and so on. Even if they knew it would be just a bunch of arguing over who should play, and so on. This is a coach and coach and manager are pretty much the same thing, except manager is a term applied to business's, while coach applied to teams. But you can still call the team coach the team manager and still stand correct.