The spring provides tension to snug the belt up. Without the spring, setting up the belt and checking timing (turning the crank with pulley bolt loose) would be difficult.
If in doubt download the Gen_3 Camry service manual at the link below for the 5SFE.
If you look hard there might be a small plastic plug in the front of the timing belt cover. On some make of cars, the timing belt is snugged up again in a certain amount of miles by removing the plug, loosing the bolt, rotating engine and retighting bolt.
Again some owners push the pulley hard against the belt (over coming the spring tension) then snug the bolt down. They state this reduces belt slap.
http://oregonstate.edu/~tongt/camry