If you complete every multiplayer mission in the game (which is to say, you have to 100% each singleplayer mission with all of the chests and treasures obtained to unlock them in multiplayer), you gain a new ability that you can apply to your leveling grid to greatly increase the size of your limit bar. Basically, it just lets you use your Limit Break almost immediately. Unfortunately it isn't any new power-up or new ability, but it makes things a lot more exciting, as I'm constantly using Riku's Dark Aura Limit during missions with it. That said, it isn't a very big deal, unless you're a big fan of the game, and wanted to collect every possible unlockable.
