I've always remembered a quote from Jimy Williams, former Jays manager. When he was hired, he said "Managers can't win baseball games, but they sure can lose them." The gist of it is that, if the players are all playing to their ability and skills, it doesn't matter who's managing. Because if they're all doing what they're capable of, in the roles and situations to which they're best suited, they'll get results. But it's the manager's job to know what roles and situations his players perform best in. So while he can't control what they do once they're out on the field, he can affect the team by putting them in the best position to succeed, whether it's by position on the field, in the line-up, in pitching matchups, roster management (to a lesser degree), and in handling the social management aspect of the job.