Think of Religion as good for diplomacy and your own people. On the domestic side, the more religions you have in your cities makes people happier under certain Civics (this is without even having built any temples yet). In terms of diplomacy, you get a bonus to your exchanges with other civilizations who's state religion matches your own. That generally means they'll be friendlier towards you, offer you better deals, and not declare war on you (note that all that can be offset by other factors).

I'm still getting the hang of it myself, but based on the manual and gameplay, that's what I've gleaned so far.