Most things already covered, on an NA we can sharpen the throttle response, remove flat spots and generally make it feel more alive. We also raise the rev limiter to give you that extra few hundred RPM at the top end... If you look at paper gains, it's a disappointing read but in reality to someone who loves their car, it's probably money well spent.
I do have to agree though, if you are going to compare to forced induction tuning then you are likely to be disappointed.