Let's suppose you release PDF documents such as newsletter, minutes, agenda, reports and so on every week month. Then in 1 - 2 years you'll see there are lots of files in your Media Library like the following. So do you want to keep your Media Library clean by a simple way?
You want to keep them clean but worry about it will break the link to the document in any of your past campaigns. You want to update an old document but you have to cost several minutes to find out the right one to be updated.