Added security, for whatever reason someone tries to make you unlock it and it doesn't with your "usual" finger. Makes it looks broken instead.
Or, and probably the most likely use case. If you have family that you let use your phone but don't want them to access your work/private stuff. So, only your fingerprint will work.
Similarly I think you should allow different fingers for use of unlocking the device, and making payments. Kids should be allowed to unlock the phone if they want to play a game or watch YouTube. But should they click on something like an in app purchase or a movie rental. Their fingerprint won't authenticate the purchase.
