The primary reason I use a light weight footprint(6' X 8') is to keep my tent dry and clean. With out a foot print I would be stuffing my tent with a wet and perhaps muddy bottom into a compression sack. I just tuck the access under the tent so that it does not collect water during a rainstorm. Unless you are in the desert, the part of the tent or the ground cloth will collect moisture on the bottom side even in "dry" weather. I would like to keep a relatively dry clean tent clean and dry if possible. It is a lot easier to pull out a light weight sil-nylon tarp that I use for a ground cloth during to day to dry out than it is a tent. It also makes a nice floor for the vestibule.
I've also used our ground cloth as a shelter while waiting out downpours and hail storms. It is also a good cover for the eating area if needed in wet weather.
I even carry the groundcloth when I actually have to carry it!
