You don't need to do it often. Only when you see something that doesn't make sense, or after a massive upgrade, like 1.5 to 2.5.
In rough lines, these folders contain a precomplied cache for all the shaders. So it's a cheat sheet for the code, to speed up things.
When the folder does not exist, the game regenerates the cache again from scratch, which take a while (5-15 minutes)