Teams try to win games in order to draw fans. Obviously, not everyone wins enough, so the teams need to provide other means of entertainment. This may range from a decent baseball team, to a nice stadium, variety of concessions, mascots to entertain the kids and t-shirts shot from a hand cannon.
So, Gallo fills a portion of that secondary entertainment. People love homers and want to cheer for the guy who hits the 400 foot blast. Gallo fills that role, although not often enough for most of us. The average fan (kid or adult) doesn't really get that involved in the myriad of stats measuring Gallo's effectiveness. They just want to see Gallo take a mighty swing and send the ball into the stands.