Here is the scenario. You build a house. Suddenly you figured "hey i should create a basement". You break some of the floor. and start digging your way down and dig out a space, line the walls, create everything and done. You head upstairs and suddenly it dawns on you the horror. Your house now has a gap between the ground and your house from your clearing.
No amount of flattening fixes it. So you add dirt back on the outside and prettify everything...