Field managers are responsible for writing out the daily lineup and making in-game tactical decisions (e.g. pitching changes and decisions regarding pinch-hitting, pinch-running and defensive replacements). Often viewed as a face of his franchise, managers also play a crucial role in interacting with the media before and after games and managing the atmosphere in the clubhouse.
Managers are often former players, as said experience can help them relate to current players by virtue of shared experiences and familiarity with the innerworkings of the clubhouse setting. But even with this in mind, managers still take input from the front office -- scouts and data analysts -- in an effort to make optimal game-management decisions.
Managers -- also commonly referred to as "skippers" -- are also responsible for issuing instant-replay challenges, and they will occasionally argue with the umpiring staff in protest of a call on the field. Managers are known to insert themselves between an upset player and an umpire in order to prevent the player from being ejected from the game.