Its such a simple exercise for the race organiser (as an organiser of countless events myself ) just another tick box on the registration form.As already stated its not about the prizes at all but simply being able to compare yourself to someone close to your own age. Maybe in the younger age groupings it isnt as important but as you get older ( 55 60 65 etc) your fitness, ability etc is more difficult to achieve/maintain so a closer comparison is essential.