Our appendix is useful as the last person said. However, if we evolved from apes, then the apes should have the same function for it as we do. This would help support the theory since it would make no sense for the appendix to change function just as eyes are still used to see, noses still used to breathe, etc. It's not like going from using hands to climb trees to now writing and typing and the many other things we do.
The appendix changing function is equivalent to the lungs switching up their game to digest food instead of breathe air.