Code is different. There are many times that I have been developing and I do something to my code that causes mystery behavior, and I don't catch it until perhaps days later. Being able to walk backward in time and see when the problem first occurred is invaluable.

I used to really like using Visual Source Safe. It was so easy to set up and integrated directly with Visual Studio, there was no excuse not to have a VCS. Too bad it had a reputation for garbling itself (something I never had happen as a single dev).