Bench coach? They're just making stuff up now.
Phil Taylor, CNNSi
The bench coach is a relatively new phenomenon in Major League Baseball. As the name aptly describes, the bench coach's primary duties are to manage the bench - that is, decide when to send in pinch hitters, pinch runners, defensive substitutions, and other personnel decisions. While the manager is usually concerned with the game on a pitch-by-pitch basis, the bench coach is more in charge of knowing the other team's weaknesses and then exploiting them as best he knows how with the players he has.
It has been noted that the bench coach usually has the secondary job of keeping the clubhouse together as a cohesive unit. Sometimes this is more difficult than it looks, and many a bench coach has lost his job due to in-house dissent and fighting. On the other hand, the bench coach is usually the first in line for a manager's position when it becomes vacant, due to his knowledge of the players and the situational aspects of the game.
Some notable bench coaches include or have included Ken Macha with the Oakland Athletics, Don Zimmer with the New York Yankees, and Bob Melvin of the Arizona Diamondbacks. In the bench coach's defense of their usefulness, I can only say that if the organization is willing to pay money to have another rational and intelligent baseball mind in the dugout, then that's their prerogative.