What purpose is the D on the edge of the eighteen yard box?

Answer 1

The D keeps players 10 yards from the Ball when a penalty kick is taken.

Whenever a free kick is taken, the defending team must always be 10 yards away from the ball. This includes penalty kicks. The penalty spot is 12 yards from the goal line. The penalty area is 18 yards from the goal line. All players have to be outside of the box when the penalty kick is taken. Without the D, the players standing behind the penalty taker would only be 6 yards from the ball. So, from the penalty spot, a 10 yard radius is drawn and marked only outside the penalty box (so it ends up looking like a D)