Reasons You Should Use Source Control

• You can make reductions. If you wrote up a fifty-line function with airtight, ironclad conditions to do something that you later realized you could do with a couple math operators, with source control you can just delete all of your code and have it still live in the previous revisions as a monument to your Herculean effort. Without versioning, functions like this tend to stick around out of a reluctance to discard hours of thought and labor.
• You can't screw up your old versions. If you're just doing ad-hoc file-system-based backups, that can fall apart extremely quickly as you attempt to fix mistakes made in old versions by modifying your files for that version, either because the files for that version were saved wrong or because you didn't notice you weren't operating on your working copy.
• You can branch changes off and merge them in separately. When you're using a version control system like Bazaar, you can branch every time you want to make several revisions for a single change, and continue to make other changes without worrying about it everywhere else.