It seems to me that most people have covered the major feature of source control but one of the biggest positives is skipped over. These are:
Branches
Without a source code repository it is impossible to create branches (or copies/stream/etc.) of your code for particular purposes. Not being able to create and merge branches is one of the biggest things that disqualifies VSS from being a real source code control system. Some of the purposes of a branch include:
Bug Fix
Sometimes you need to resolve a bug and do it in a place away form the mainline or trunk version of your code. This may be to resolve a problem in the testing environment or any number of reasons. If you have a version control tool you should be able to easily make a new branch (something VSS sucks at) to fix the bug and be able to merge it back into the mainline code if necessary
Maintenance Release
This could be much the same as a bug fix but done after code has been released to production. Examples would be for fix packs, service releases, etc. Again, you want to be able to merge the changes back into the trunk if necessary
New Feature
Sometimes you need to start development of a new version while maintaining your current code. For example you release and maintain v1.0 but need to start work on v2.0 while maintaining v1.0. Branches help resolve this situation
Tagging/Labeling
Another thing source code control systems do is make snapshots of the source code at a particular point in time. These are called labels in VSS, tags in subversion, etc. By creating these on a regular basis and linking them to some substantial milestone in your project it then becomes possible to determine what exactly has changed in your code between releases. This can be important for auditors but also in tracking down the source/scope of an issue. VSS also gets a fail here because VSS only versions the files, not directories. This means it is impossible to re-create a previous version of the system if you rename/move/delete files or directories in the repository (something that happens a lot if you refactor). Good source code control systems like Subversion do just this.
