The other, being REW... is more like a magnifying glass... the resolution you can see with allows you to better judge what may be happening.
A good example:
REW allowed me to see better where I had a 20dB dip in my response. With a little checking, I found the wavelength of that frequency was exactly the distance from the speaker to the wall and back... thus creating the cancellation or suck-out. Once diagnosed, I was able to adjust by moving my speaker about 1.5" and corrected for that problem.
XT32 allows me to set the maximum correction frequency to any point I choose, which is just below 200Hz in my room, where the Schroeder Frequency occurs. The Schroeder Frequency of a room is different for every room, but will usually fall between 200-300Hz. This frequency is the point where the room stops acting as a resonant chamber. Below that frequency, most rooms need help. Above that frequency, most rooms tend to have a much smoother response and you might only have to make adjustments due to unusual factors.
Together, Audyssey XT32 and REW can be very powerful. Audyssey by itself is somewhat limited. REW has many other features, but on its own, cannot do what Audyssey XT32 can. You would need other equipment still to replace Audyssey completely.
I hope that helps you understand better.