Normalized Power is not a measure of power, it is an expression of power variability in a workout, used for the assessment of training load and intensity.

It is the exact purpose of NP to reveal how a workout was done, i.e whether it was steady-state efforts or included peaky, high power, by comparison to average power. You could go in and look at a ride file or use graphs to see the same thing and get an understanding of the physiological cost of a ride, but having numbers like AP and NP make that really easy to do.