In the past I have seen some tent footprints that have grommets for the poles so that you can set up the tent fly with the footprint only (to save weight when you do not need the tent, but want to use the fly as a "tarp". This is the only reason I can see to buy a pre-made footprint. By the way, putting in your own grommets is not hard to do either.

It seems to me that really essential gear is seldom on sale. There are always buckets of tent footprints on sale. Makes you wonder.