I'm guessing, but I would think that the idea is to have all the junk in one place so that it can be safely cleared at startup and excluded from backups. If th

I'm guessing, but I would think that the idea is to have all the junk in one place so that it can be safely cleared at startup and excluded from backups. If the user tmp files were placed in /tmp/${USER}/ then that would achieve the same goal.