- Track changes to your code base (i.e. who changed what), and roll back to previous revisions
- Collaborate with your teammates by having the version control system merge your changes with changes made by other developers on the same file(s)
- Create branches of your code, allowing you to maintain different versions of your project in parallel
You can use subversion directly from the command line, but there are also GUIs and IDE plugins that make subversion integration easier. AnkhSVN, for example, is a subversion client that integrates with Visual Studio.
I recommend you do some reading on version control, then follow a few subversion tutorials to get you started: