As all the above - mainly protection online (if a fraudster gets your credit card, they may spend some of your credit limit, but at least you still have your bank account to use while it's being disputed).
Also the perks.
Tesco Clubcard gives clubcard vouchers, that I turn in to Virgin airmiles.
I have another card (not a credit card though) that gives cashback, so I will often keep that account low but when I'm doing a big purchase/payment I transfer money to that account to make the payment.
We bought something the other day that cost £1,300. Transfer £1,300 from one account to another, spend the money and got £13 back in cashback!
If you're going to spend the money anyway, you might as well get some extra benefits.
It also keeps money in your own account for longer, so potentially earning you a better amount of interest.