Elite nights are those nights that count towards earning your elite status. You need 10 to achieve Silver Elite status, 50 for Gold, and 75 for Platinum. The most common source of elite nights are paid stays. There are however other ways to gain elite nights such as reaching various spending levels on the Marriott branded VISA card. I think it's currently 1 night per $3000 spent. To answer your question, you don't use them for anything. It's just for determining your status level.