Shocking the pool will eliminate algae, combined chlorine (CC), and any other organic contamination. If you have algae, or the (CC) level is above 0.5, or the free chlorine (FC) level is zero, you should shock the pool. Many people also like to shock the pool when the FC level falls significantly, as it often does after a major storm or large swim party. This is done even if FC doesn't get all the way to zero, just to be sure everything is all right. If you are maintaining proper FC levels there is no need to shock weekly, or on any other regular schedule.