Rain can affect certain skills, monster abilities, drop tables, and spawn tables.
Last 30 minutes
Sunny is a 4:1 ratio to rainy
Rain and other forms of weather can affect the environment by filling areas and not allowing passage by the players. Example given was rain filling up a sewer passage to the castle that players can use as an alternate assault path when it is sunny. This means the few players that have the ability to change the weather to rain could change it to rain for 30 minutes and block that passage.
Example given for a player spell that is affected by the rain is the lightning spell will bounce to multiple targets in the rain, but only hits one target when it is sunny.
It also sets the ratio of sunny and rainy to 4:1. You never know when the weather will change, but once the weather changes, it will last at least 30 minutes. It is said that it was intended to give a moderate tension when the predictable area and the unpredictable area were properly mixed.