Why does Starship belly flop ?

I was wondering why spaceX make the Starship Belly flop during i was watching everyday astronaut 

but what a coincident i just find out that he do have a video about why. And my guess is it might save some energy for the ship because rocket fuel cost alot but it is also a risk because if the calculate was wrong we must pay lives for it . Now come watch with me this video to understand why : https://www.youtube.com/watch?v=BqJ5bKuApbs