Interest rates.

Paying it back immediately still means that you're paying extra for everything.

The only thing it does is boost your credit score, if you pay it back on time.

It also usually implies you're living beyond your means, which isn't sustainable unless you know that your income will continue to increase and you can manage to keep your spending the same. That's rarely the case.

The credit system is designed to enable bankers to keep increasing their profits and everyone else just throwing money away.

It's a scam.