What you end up with is a nice pouch with a complete sheet set. The pouches are easier to stack in the linen closet, easier to look through and grab what you need, and your sheets and pillowcases ...

The top sheet you sleep under is usually considerably larger than your duvet on top, leaving excess slack that needs to be tucked in for your bed to look presentable.

You take the flat sheet, pull it up over the top of the bed and tuck it beneath the mattress. Then you smooth out the flat sheet and fold the bottom up just below the pillow, making a pocket of sorts.