They add a bit of spice to the game, no 2 do the exact same thing. Some give new PvP methods and opportunities, some emphasize co-op play, one gives you something for PvE. Most of these give you unique spells and gear and some of the spells become unusable if you abandon the covenantAnyways, what is the point in the Covenants? And can I join more than one? Love the Pharis-bow btw, that made my heavy dext-leveling finally worth it!
You can join multiple but not at the same time. You'll lose the benefits of your previous covenant, and any rank progression you made is cut in half