Brandon Marsh wets his hair repeatedly during games primarily because he prefers the way it looks and feels when it is wet. He has said that if he doesn't wet his hair, it gets "super bristly," which he doesn't like. Marsh usually pours water on his head multiple times a day, including during games, to keep his hair slicked back and manageable. This routine started years ago when he was with the Los Angeles Angels, suggested by his teammate Michael Lorenzen, to help manage dry hair and style it better. Marsh does not use hair products or conditioners, relying simply on water to maintain the slick, wet look that has become his trademark style on the field.

In summary, Brandon Marsh wets his hair to avoid bristliness, to maintain a slick, styled look, and because he likes the aesthetic and feel of his hair when wet. This habit has become notable enough to be part of his identity as a baseball player.