Not that interesting really, just real estate. But like many of these features that don't initially seem much practical use, when you start thinking, or seeing what others are doing with them, some can really make nice differences. I recently got a WFT to take over from the EyeFi card I got so frustrated with, boy what a difference, never saw the value of a $500 option like that before but I wouldn't be without one now, though I was lucky and got a secondhand one for $160.
Anyway, nothing I do with the REL button on a 600EX couldn't be done with a $25 Yongnuo RF-602 (which is what I used to use) in addition to any remote flash setup, but the point of the RT system is that it is built in and just works. Cuts down on lots of remotes and batteries too!
My normal setup is this, it is next to impossible to light an entire above average sized property really well with less than fifteen or so small lights. So what I do is take fifteen or so shots and move one light and then blend the results. I have my camera set up on a tripod with the ST-E3-RT and SR-N3 connected, I then walk around with a 600 and point it at the building. When I want more lights than are available inside to shine through windows etc I will set up another 600 or two to do that at the same time, it just saves some time.
Neuro's idea of using it with a key light during portraits is also very good, and I will do that, if you frame loose and can activate the camera from a distance talking to or interacting with the subject I'd expect some interesting results, people are certainly much less stuff when you are not looking through the camera.