The lack of flow will turn off the SWG, so that part is correct. But, what if the flow switch goes bad? Is a current thread around here of someone just recently that had one go bad and resulted in showing false flow. You could end up with a home made pipe bomb. The timer gives added protection of it not even TRYING to run when you know the pump would be off.
You may also find you want the pump to run longer and not the SWG, or turn the pump on when company is there but don't want the SWG going. Gives you some flexibility.