Why does my iPhone have a SIM if I use Sprint?

Supposedly, CDMA networks don't use SIM cards. But I just now opened my SIM card slot and there was one? What's up with that? Btw, I still have service without it, I just found it strange that it actually has a sim.

Do you think I'll get charged anything on my cell phone bill if I take it out? I keep getting a "Invalid SIM" alert but if I take it out completely, it works fine.

All iPhone models -- with the exception of the iPhone 4 (CDMA) -- use SIM cards.

Unless you have the CDMA version of the iPhone 4, your iPhone includes a SIM card.

It's included because it came with the iPhone. It's yours. Just take it out and save it for future use.

And if the iphone is unlocked that's even better, cause you can use almost anywhere

All iPhones have a SIM slot in them, and all iPhones will come with a SIM card inside.

Just because there's a SIM card doesn't mean the phone uses it.