The arc at the top of the penalty area is not a full half circle. It is scribed out with the penalty mark as its center, and it's there to provide a reference mark that allows players and the officials to know where "10 yards from the ball" is with reference to the penalty mark. Upon a penalty kick, players will take up positions about the top of the penalty box, but outside the arc. The top center of the penalty box is actually a bit closer to the penalty mark than 10 yards. Hence the need for the line. The Laws of the Game require players to be ten yards from the penalty mark (and outside of the penalty box) at the time a penalty kick is taken. Players must remain outside those areas until the ball is touched to (re)initiate play.
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The semi circle purely serves one purpose. It marks the distance away from the ball the players must stand when a penalty kick is being taken. Close observation will reveal the center of the circle to be the penalty spot.
The semi circle at the edge of the 18yrd box is there to keep the players 10yrds away from the ball during a penalty kick. No players are allowed in the semi circle until the ball is kicked during the penalty. During normal play the semi circle offers no use. It is not part of the box so the goal keeper will commit a foul if handeling the ball in the semi circle.
Because when a penalty is taken all the other players cannot be inside this area or in the penalty box itself.
During a kick off, the opposing team may not enter this circle until the ball is touched by the kicking team.
When restarting play, the other team can not be inside the circle until the ball is touched.