Shocking is done by taking their chlorine to the upper most limit based on the amount of stabilizer (CYA) in your pool to kill anything waiting to take hold. Why does that happen? A heavy bather load can stress your chlorine to the bottom limit of sanitation or even lower. When that happens, the nasty stuff is not all killed by the chlorine.

I only shock after a heavy bather load by my daughter's friends. I will also shock when we get thunderstorms for a few days in a row. Wind and rain can also introduce nasty stuff into your pool water. I would also shock my pool if a little child using those supposedly water-proof diapers were in my pool or young kids I noticed staying in the pool almost the whole time and never asking to use the bathroom. When testing my water with the FAS-DPD, if I notice any Combined Chlorine, I shock that night.

Mandatory shocking is required if you find a dead animal in your pool or evidence of fecal material or on rare instances, someone can throw up in your pool.

Shocking will keep your water safe for bathers. Again, some like to do maintenance shocking (because they don't experience a heavy bather load) and others shock when they know they need to based on use.

I hope this helps.