I'm going to start making some valve amps as a proper venture so this is my first main tool investment.
It does a bunch of fancy little things. Being able to graph frequency response (eventually) should be handy and you can't see distortion that easily on an oscilloscope so doing measurements while knowing how distorted a signal is should be good.
So, in terms of a full system like an amp, that is interesting to me. However I've also thought about making a little rig and just putting a signal through a single capacitor and seeing what happens. say using a few different 0.022uf caps and seeing if the response to different criteria changes. Could try a similar thing with valves. Could be a few interesting things to do with it. I also want to get a Frequency Spectrum Analyser.
It's just about being able to quantify things for myself and about having the means to do real tests, instead of just saying "this type of 12AX7 is warmer".