The Thrill of Competing
If a person is not one to follow any sport on a regular basis, they might not understand the thrill of competing in even amateur events. Working out could be nothing more than an onerous chore to them, but some who have been forced into exercise for health reasons might suddenly catch the idea. Competing against others lends a bit of excitement to their workout if they know they can do well, and it is a good motivational tool for anyone who likes to win.
There are no guarantees that a person working towards competition will be able to win a prize, but many might find their own accomplishments are the real reward. Their ability to do something they never imagined before can help them stay with an exercise program long after they might have quit before. It is a good way to help them see there might be many benefits to staying the course.