It's the best way of checking to make sure the car is performing as it should. It's not just about figures.
For example last time I was at the RR some guys turned up in an astra that was kicking out 30 BHP less than it should of. The cat had blocked up.
2 weekd later they return with a new cat and then kick out 60BHP more than what they originally had (this is since he bought the car)
A perfect example of why.
