To explore how a higher power interacts with devoted followers. To explore themes of religious devotion, community, and conflict. To explore the distinctions between the gods, and why those distinctions may actually matter in the setting. To ensure that characters for whom the deity is the single most important aspect of their existence can interact with the world in the unique manner that conviction demands. To mechanically explore the implications of the above, by means mundane or magical, given a thematically distinct source (whether that is pure faith, the largesse of a higher power, or even the manipulations of a third party).
In short, to make sure that somewhere in the game "deity" isn't just a line on a character sheet. The traditional role of the cleric as the sole healer is a weakness or possibly charming quirk of the game, but its the people who play them for other reasons that ensure healing is not their sole raison d'être.