Sometimes when you install a new OpenBSD snapshot, something is broken and you want to return to a previous snapshot.
If you are lucky there is still a mirror who hasn't synced to the latest version. Or perhaps you still have the install files from the previous snapshot somewhere.
But I wonder, is there perhaps some place on the internet, where old snapshot versions are archived? Some mirrors are still hosting extremely old versions of OpenBSD which probably nobody will use for production anymore, so a server who archives snapshots doesn't sound that unreasonable to me.