The reasoning is that you want as much force as possible moving the slide forward to successfully strip a round off the magazine and chamber it properly. The more force involved in that operation, the more likely it will be able to "push through" any problems it would otherwise catch on. This is especially important for hollowpoints. Likewise for ejecting a round, if you don't use enough force you can get a stovepipe.
Walking the slide forward is known to induce feeding problems. Most guns are pretty good these days, and most guns will work even if you don't slingshot them, but its good muscle memory to develop that will serve you well with ALL guns.
This same logic applies to dropping the slide on a round. It is better to pull the slide back and let it go than to drop the slide release. The reason is it has more force with which to strip the round off and chamber it.
Some people worry that they are unnecessarily damaging their guns with this method. The force you exert by sling shotting the slide is far less than what happens to the gun when it fires a round.