Because it is crazy good at the track. So much faster than I can do it myself. And it doesn't upset the car. There's only the rare situation when you can't downshift -- even while steering, if it comes to that. Sometimes people hit the binders too hard in front of you, even if you select the right gear before the turn. There's a reason the higher level pro series cars don't have a third pedal.
Otherwise, there's no way I would get one on anything but a commutermobile/driving appliance. Manuals last a long time, and most likely I can do a clutch swap or a full tranny replacement in my own garage that way.
P.S. You're not a man just because you drive an MT. You're a man when you can SWAP OUT an MT. Bitch.