Rain is a god power shield, i.e. while it is in effect, no other god powers can be cast.
Like most economic god powers, Rain is best used early on to gain an edge in terms of resources over an enemy. In this case, it is best to maximize its effect by having as many Laborers farm during the power's effect. However, if a player also worships Bast, it is best to research Sacred Cats and Plow before casting this power in order to exploit the improved gather rate.
Rain can also be used later as a blocker to prevent opponents from using their god powers in battle.
In team games, Rain provides a nice boost to the economy of the entire team, especially if the player has Atlanteans as allies, as their Citizens gather resources nearly three times as quickly as other villagers.