Here are a couple of use cases that typify how you can leverage Subversion in your day-to-day development activity.
Scenario 1: After a phone call with a panicked client, you are forced to access the production server to fix an issue that is affecting thousands of users. Unfortunately, after a couple of edits, the page returns a 500 HTTP error, which probably means that you made it worse than before. Unfortunately, in your rush to fix the problem, you didn't make a proper backup of the file(s) you were editing and have no way of reverting to the previous version. For the time being, let's ignore the fact that you probably shouldn't be working on your production environment. After all, we have all done it—even if it is wrong.
Scenario 2: That open source project you started while commuting an hour on the train every day has been kicked into overdrive. SourceForge.net named it project of the month and suddenly you have several qualified volunteers that want to contribute...