Ref T4039. Long ago these were more freely editable and there were some security concerns around creating a repository, then setting its local path to point somewhere it shouldn't.
Local paths are no longer editable so there's no real reason we need to provide a uniqueness guarantee anymore, but you could still make a mistake with bin/repository move-paths by accident, and it's a little cleaner to pull them out into their own column with a key.
(We still don't -- and, largely can't -- guarantee that two paths aren't equivalent since one might be symlinked to the other, or symlinked only on some hosts, or whatever, but the primary value here is as a sanity check that you aren't goofing things up and pointing a bunch of repositories at the same working copy by mistake.)