Well, they'll have a lot of food, which allows the culture to multiply more, and into the mixture that you just added. You are trying to dilute all the byproducts that are left in the culture during dormancy. When you feed then dump, you are dumping out potential food for the culture. It is still diluting the culture of course, but why waste the new flour? You do feel strange dumping a bunch of starter out in the beginning. But when you start trying to keep it, you start filling up the fridge with jars.