Depends on the tent, I love my Hilleberg footprints...they cover not only the tent bottom, but all of the vestibule space as well!
Footprints do more than just protect the bottom, full coverage prints also greatly reduce moisture rising up from the ground.
I know they are not needed, but once you have this "luxury", it sure is tough leaving it at home. Great for entering the space, pulling off shoes, storing gear without setting in the dirt/mud, setting up the dog's bed, etc.
Here's a shot of one: