What is the point of a flat sheet? point of a flat sheet for beds?

point of a flat sheet for beds?
I'm looking for wallpaper--the quality that go on...
The number of threads per inch.

Ikea - ?!?!?!?!?
The flat sheet is used as a cover for the blanket that goes higher than it , so you are not in contact with the blanket, more so if its woolen, or made of synthetic material like the soft fleecy ones.So you have your mundane bed-sheet , then the blanket and over the blanket the flat sheet, the flat sheet then fold over giving the untrue bed an extra sense of comfort!
So I am going to paint my room...
The flat sheet goes on top of the fitted sheet. You sleep lower than the fitted sheet. It keeps your comforter or bedspread cleaner longer.
Room color scheme?
Answers:    Actually, you sleep UNDER the FLAT sheet. The FITTED sheet is the one that fits TIGHTLY over your mattress pad to protect your mattress. The flat sheet is of late for layering(nice to toss your bedspread/comforter to the end of the bed on hot nights), and the flat sheet that you sleep UNDER, also protects your bedspread/comforter from getting dirty...