Ignoring the improved power/torque/economy/throttle response/speed delimited reasons for now...
One of the main reasos people get this done is to remove the 1st-3rd gear throttle limitation. Some people say the car only allows a percentage of the throttle in these gears but I've been told that's not exactly true. I was told that it's like an elastic band on the throttle, so if you push the pedal to 100% open, you'll receive 100% throttle but it will be a slightly gradual increase to 100% rather than immediate.
While this sounds like an obvious improvement for a sports car, I also found that it also has a downside in that if you use your car for in town/stop start driving you have to be more precise with the throttle as the car won't smooth out your actions. It wasn't an issue for me and I'm sure you'd get used to it quickly anyway.
HR still benefits a lot from an UpRev.