I used this for developing an audio recognizer (https://github.com/trishume/PopClick) and it was incredibly cool and useful.

You can write your own recognition and analysis plugins pretty easily and then overlay those on the spectrogram so you get a sense of what your program is doing and why it is going wrong. I don't think I could have ever successfully gotten a recognizer working if I hadn't found Sonic Visualiser. It's awesome.