You need it to see the true ending. If you killed the Time Devourer in the final battle without using the Chrono Cross, the game just cuts to the credits, you don't even get a proper ending. Luckily it's only for the actual final battle. If you want the extra endings, you can just curb stomp the Time Devourer which is pretty easy to do since he's a bit of a joke. The Dragon God is more challenging but frankly I'm with some fans who feel that Miguel is the true final boss and the rest of the game is a cakewalk after that except when you fight Dario...