The possibilities are endless and the best use I've found for REW so far is guiding me on exactly which room treatments, how many, and where to place for the best results, and to stop focusing so much on the frequency response and realize that is only about 10% of the picture. Once I improved my modal ringing/decay times, my ETC, group delay, comb filtering went away, things tightened up, and my system now sounds as good again as the difference between no Audyssey and with Audyssey. It is truly refined now and that just cannot happen without measurements and understanding them.
This is one of my main areas of interest too. I have treated my room and the result is terrific, but I would like to learn more about verifying the results I am hearing.