The way its used to tune an engine is that the operator can hold the engine at particular points in the rev range and monitor what effects changes have, or go up and down the rev range and monitor it.

Its just a way of allowing the engine to be worked though, from a tuning point of view the techniques are the same wether its rollers, dyno, live map essentially, you basically start safe, and keep adding a bit more timing till you hear Det, then you come back a bit.
Fuelling is slightly more complicated as its not just about making peak power its also about managing temperature as well, often the most powerful point on a turbo car is too lean to maintain safe temperatures so a good tuner will run slightly richer and sacrifice a few bhp in the interests of reliability.