I was ninja'd when it came to mentioning the Movie-based games. Anyway:
I don't see any reason for Pokemon Crystal to have existed. Same for Platinum. They only served as filler for fans hungry for an actual new game. Dissida is another game which didn't necessarily need to be made. It was a great idea and I enjoy it but, I don't see any reason for it to have actually been made other than fan-service and even that was a bit messed up as there are people lamenting the fact that it doesn't include Lock or use the Onion Knight's actual name...