You need to shock to burn off combined chlorine--chlorine that has been "used up" by sanitizing gunk in your water. So when you have CC over .5, or 0 CC but visible algae, or cloudy water, shocking is the first step to getting it cleared up. If the pool chems stay where they need to be, all the time, you'll rarely have to shock.

That being said, I also shock after a large bather load or after a bad rainstorm.

Janet