Short-term storage is useful for things like being able to make 5 photocopies of a document--- old copiers would have to literally run the copier across the document 5 times to do so. Now they're more like scanner->printer, so they scan once and print 5 times. They don't delete it immediately after the job either, because many copiers support convenience features like "ok, 2 more copies of that last document please". It does seem like they could delete once N more documents have been printed in between, though.