I'm not a staff member, but a user like you, with a couple of answers at least.
Once restored, a copy is kept in the chest in case you make a mistake with the file or the restore fails somehow. Once you confirm that it has worked correctly, said files can be deleted from the chest.
Delete is not the best idea, as may be obvious now. Unfortunately it can take a mistake for these things to become truly apparent to everyone...AFAIK there are ways to recover previously deleted files, but I would not be the one to ask on this subject.