Great question. The reason is that my documents do get updated. This is not merely an archive as I also store various more live documents in it like budgets, etc. Some documents are indeed purely archived. But even for those, I find it really handy to have a complete log of when they were added. Who added them, etc. All this is stored in SVN. Also, SVN provides a really easy way to sync the folder to various devices (svn CLI tool on Linux and Mac, and TortoiseSVN on Windows clients).

I am aware that nowadays, SVN is not the modern choice compared to Git. When I designed this system, Git was not available. But I actually find that SVN's centralized architecture fits really well for this purpose.

For photos, I really like having them in SVN as I never lose any image even if I modify it. Sure, it takes up a little extra storage. But storage is cheap. Also, SVN here acts as an extra local backup.