Why do civilians need high capacity magazines?

My friend and I were having a casual conversation about guns. He's a liberal and not gun friendly. He mentioned that he thinks people should only be allowed to have 10-round magazines and shotguns. He said that I don't need 30 rounds in a rifle to defend myself, and that if I can't defend myself with 10 rounds then I fail. He also mentioned that he doesn't see the point of a 30-round magazine for sport-shooting.<br><br>So, my question. Why do civilians need high capacity magazines? What is the reasoning behind this?