At my day job, we spend a lot of time thinking about and talking about the advantages that version control bring to all sorts of digital data, so much so that the company's new tagline is: Version Everything!
Here's a nice essay about exactly why this is so important, and how simple it can be to include Version Everything in all your work: Deploying PostgreSQL Stored Procedures.
Months later, the function needs to be modified again, this time by someone else, who makes the change to the function source code file in the VCS, commits, tests and deploys the function.
Whops! The local modification made by the evil consultant is gone! Alarms goes off, people are screaming “the system is down!”, people are running around in circles, you revert your change, back to the previous version in the VCS, but people are screaming even higher “the system is down!”.
Nice work, Joel!