The phrase "You have to start somewhere" comes to mind. A CS degree is a more formal and substantial pursuit than what you may have picked up doing it yourself. Could you code without a degree? Of course. Are you very likely to be a better developer after completion? Yes.

I think a lot of it comes down to establishing a set of tools that can be used for whatever problem you encounter. No matter the language or the problem you want to have at least a vague idea of how it might actually work. Further, when you in inherit a code base, you want to have the tools to dig in and find out how the thing works. And when problems arise, you want to be able to get inside the black box and ask things like "Does this result make sense."

A big missing piece in a CS degree is an understanding of the SDLC. The use of svn, tagging and build scripts is essential to any software development effort, and yet it is rarely mentioned. Creating elegant algorithms is great fun, but when the client is screaming that the code doesn't work and all you can say is "works on my machine", you'll wish you had the warm blanket of a proper build process.