It allows more divergence because the character types behave differently. A
priest is a priest per the current tules, and their behavior is based
on fantasy
'Chrisrian ideology' more or less (i.e. Prayers create intangible benefits,
reliance on faith, etc).
If you want a world where priest perform miracles and those miracles are
directly tied to the gods (so a war priest and a storm priest do different
things), then you have to jiggle the system somewhere. Further by
creating those
concepts and tying their benefits to mechanics you create more divergent
characters because they are quantifiably different.