Some players self-motivate. Others are motivated but need direction in what to do. Others are complacent. Insofar as they are coaches, managers can undoubtedly help many players during games.
A manager also has a wider persepective of the game, which means players will not always see the problems happening across the pitch and react accordingly. The fact that every successful manager in the modern game does it, tells us that it has an effect on the team.