on O/U you can catch the shells easily, same with pumps, how ever with autos the shell flys out (obviously) and when your shooting good ammo like Winchester AA that run about 7 bucks a box, you want to keep the hulls because they are made very well and the serve for one of the best if not the best hulls to reload with. I dont reload my shells but a couple guys on my team do and I give them my hulls for there home loads. Another reason for a shell catcher is that when your shooting trap say the guy next to you has an $8000 kriegoff, you definatley dont want your shells hitting his gun when you fire, so out of courtesy you have a shell catcher.