The biggest reason a baseball field is in the shape of a diamond is to benefit the umpire!
Baseball diamonds are really squares tipped on their sides with the tip of the diamond the home plate. The game of rounders set the stakes up in a similar pattern, but it was Alexander Cartwright who decided the baseball diamond should be set up exactly so, with equal distances between each base and the home plate.
The baseball diamond is shaped in this fashion for a couple of reasons. One, it is logical that the runners should cover the same distance between each base, allowing for each hit to count for the same potential distance for the runners. The biggest reason for the diamond shape, however, is to benefit not the players, but the umpire. The unique diamond shape of the field allows the umpire to have a superior view of the entire playing field from behind the catcher, affording accurate calls throughout the game.