because its better you fucking retard.
JJ, your theory is correct. i had the same theory in a previous thread about case savers. its always best to run the biggest sprocket possible, to minimize the space between the chain and case saver. eliminating the "balling up" effect of the chain will help to eject it cleanly as opposed to it tearing the shit out of everything.
i run NO case saver and i can safely say i would run nothing over a billet one, but thats me. if i was able to run a case saver, it would be plastic. my front sprockets are just too big for me to run one.