How many do you have?
You hear statistics that the average person has 3 credit cards, or whatever. Why? I have one, and don't feel a need to have another. So why would you have more than one credit card?
If you have an issue with one credit card, you have a backup. Say your card number was somehow stolen and someone charges on it and maxes your credit card while you're on vacation. You go out to dinner and they reject your card. You have another one to use in the meantime while you figure out what the heck just happened.
I've already had an instance where a bank shut down my card because the information was "compromised". They sent me a new card but I think they told me to stop using my current card, so I had a few days where I really didn't have that card. I know other people who have been without a card longer than that, so it's nice to have a backup.
My case happens to be that I'm young, so nobody wants to give me TOO much credit yet. It's helpful for me to have a little more spending power than I would with just 1 card, especially since I can afford it. Visa is always a good backup card since they're accepted everywhere, but for rewards and some of that other stuff, I don't think they're the best...so I go outside of Visa for my primary card.
That's all I can think of, anyway.