Make a bed means to move the duvet/top sheet so it covers the whole bed.
Reasons that you'd want to do this: hygiene (air doesn't get where you sleep - both a good and bad thing), and design (looks as it should).
Reasons that you'd want to do this: hygiene (air doesn't get where you sleep - both a good and bad thing), and design (looks as it should).
Meanwhile, design is a circular argument. It only "looks as it should" if you think you should make the bed.
I agree with you, which is why I personally fold down the top half of my bedding.
As for design/looks I likely described this wrong. Bedding is designed to be made full on the bed. It's not a circular argument. The design is best when the bed is made as you can see whatever design is on the pillows, sheet, etc.
Another reason: it's a small accomplishment that could be a good motivator for your day (especially if you're depressed).