It's said so often it seems like a cliché that can be ignored, but it's true: The mental game in sports is half the battle. Keeping yourself sharp psychologically might not improve hand/eye coordination or make your slider dip, but that doesn't mean it won't have a measurable effect on your game.
Consider the case of Chuck Knoblauch, a Yankee's second baseman, who suddenly developed what's known in baseball as a case of the "yips." For those unfamiliar, "the yips" is a term applied to many different sports, and it basically means one loses the ability to effectively play the sport. In Knoblauch's case, it meant he suddenly couldn't make a throw to first base. (Rather important for a second baseman.) Rick Ankiel and Steve Blass are also examples of Major Leaguers affected by the yips. Ankiel, a pitcher, had to become an outfielder to break the spell, and Blass left baseball.
Clearly, the psychology of baseball is no laughing matter. Using mental rehearsals and teaching yourself to control your focus is imperative to making a baseball player game-ready. So-called "smart" players -- those who have a crisp mental image of the most effective play or strategy to win the game -- are a lot more highly valued than the image of big, dumb animal sluggers that used to pervade the game.