Speaking as someone who went through the VSS -> SVN transition process for a large codebase, I would say the biggest benefit is being able to sleep soundly knowing that your SCM system wont suddenly have a hiccup that corrupts your database and you have to go back to yesterday's backup. You do backup your database daily, right?
With VSS, corruption happened at least once per month. With SVN (same hardware & OS) - not once in over two years.
Oh, and the branching/merging capabilities are sweet!
