Unlike a race in most other sports, a sailboat race starts long before the starting signal.  Assigning a slot or starting position on the line for each boat isn’t possible because sailboats can’t remain fixed in one place until the starting signal goes off.  Each boat has to sail back and forth looking for a place on the line.  A countdown to the start is used to allow the boats time to find and maneuver into a starting position on the line.  Each skipper must keep track of the countdown to time his start on the starting line at a good speed on a favorable tack in a good position.