As for keeping you drier, well, I doubt that. All modern, good-quality tent floors are coated with polyurethane, forming a completely waterproof barrier. So a floor that leaks is: A) defective, B) worn out, or C) standing in a two-inch puddle of water while a couple of 200-pound campers bounce up and down on the floor. Moreover, if moisture is getting into the tent through the floor, it will find its way into your gear, either by squirting out from under that inside-the-tent ground cloth, or by evaporating and condensing on something else.
Depart in peace, my child, and put your ground cloth UNDER the tent. Or you're gonna go to camper hell.