If I understand your question, there are two answers that are both pretty basic.
1) Lack of cash - some people just like to buy things that they can't afford. It's an unfortunate reality of credit cards. Some people may only have $50 to their checking account/debit card, but have $5000 or more in available credit to spend.
2) Rewards - for those who spend responsibly, pay off their debt monthly, etc., some cards offer 1-5% cash-back on certain items. So you could pay for something with a debit card and get nothing, or pay with a credit card and get 1% back. One percent on all of your purchases can go a long way.