The same reasons you get diffusers on a ZS etc. If it goes on an interesting/cool/winning racing car then you're likely to see it in some form on a road car.
Take the NGTC cars for example, a very open aim was for them to be styled to look more interesting.
Also, everyone knows the faster you go, the more downforce a wing produces. Cup racers go faster than you do on your way, well anywhere. However given the one make nature it wouldn't make a difference if they were removed as it'd only affect high speed stability and more to the point everyone would loose out equally... And so I return to the suggestion of it looking good and selling cars.
Why do cup racers bother with a different spoiler then? Dunno. Another tool to adjust balance for the sake of car setup?