Why does cry + es go to cries? What is the reason for having an 'es' suffix as opposed to an 's' suffix. Is it due to the pronunciation as cris would no longer sound like the base (but then we do know that words can be pronounced differently and still have the same base e.g. heal and healthy and healing.)

Also, is the 's' and 'es' which go on the end of words, is that the same suffix as the ones that make plurals e.g. dogs and boxes?