There are a couple of reasons.
1) If you stop the pump, the tablets continue to dissolve and build up a high level of chlorine in the skimmer and when the pump starts back, all that chlorine goes through the system possibly damaging equipment.
2) Trichlor is also pretty acidic and along with building up chlorine, it lowers the pH a lot and it passes very low pH water through the system when the pump is started back.
Neither of those things are good for your equipment, and over time it will damage it.