For one, it'll keep you up way late as you run around gigantic cities like Metropolis and Gotham, doing heroic or villainous deeds at the behest of your mentor. For another, it adds any gear you find to your library of appearance options, letting you keep your guy the same or update him as you wish, whenever you want. Lastly, the fighting it a lot more exciting than pseudo-turn-based MMO combat like WoW... and we can't have anything fun.
In summary, this game is pretty triable.
Oh God I can't stop playing why-