REW is just another tool in box and at the very least it'll show you the visual frequency response of how you like your setup to sound. It can be useful for speaker/sub placement, crossover settings, effect of port plugs, etc.
For me REW sits one step above the "built in" room correction many receivers have now. It gives one a visual baseline when tweaking your system for the way you like it to sound but in the end my ears always have the final word.
Steve