Or, perhaps, to allow for easier creation of stories of subversion, internecine strife, or just clerics going off and starting their own sect without the god immediately revoking powers. Or, heck, within a PC, the real-world drama of a priest who doubts or sins or starts thinking about the religion in a new way without immediate confirmation of "the right answer." Why immediately jump to "it's so people can play clerics without the obligations"?
People still play clerics because they like the fantasy of being tied to a belief system or higher power. People still play paladins because they like the idea of championing a difficult ideal. That remains true even in versions without the looming threat of power revocation.