what is the use of half circle at the top of big box?

It is called "Penalty Box Arc" or just D, the only use I know of is that when you get a penalty kick, all other players must stay outside the Penalty Box (Big box) & the Penalty Box Arc until it is kicked.

All other players apart from the taker must be at least 10 yards away from the ball when a penalty is being taken. The box itself isn't 10 yards, which is why the D is added.