Running with a GPS
It makes no sense to me. Why do runners need a GPS (global positioning system) when they race? Do they think they'll get lost? They can just follow the runners in front of them all the way to the finish line.
While it may not be apparent why runners need to know their precise position at any given time on the race course, a GPS will tell them how far they ran. If the distance is not what you advertised, then your course may not be the distance you think it is.
USATF certifies courses along the shortest path possible, making every turn on the inside of the course, thus ensuring that every runner covers at least the official distance. Most racers travel somewhere between the middle of the course and the inside turns, so they always run a little bit longer than the official distance.
If their GPS tells them they ran much farther than that, though, you may have an inaccurately measured course (especially if it wasn't certified). While the GPS users may seem a bit obsessive, they are still a valuable check on your measuring ability.
By the way, a GPS can also tell a runner if the mile splits were in the right places!