It's pretty cool.
 
 http://www.sonycreativesoftware.com/spectralayerspro/features
 
 

That is pretty cool. Sony has caught up with Adobe Audition in less than a decade.

What exactly would you use either of these programs for (or Steinberg Wavelab?) in a purely  music recording sense  - I've always wondered - is it purely to improve noisy recordings (hum etc) instead of just recording them again properly, or is there something else, perhaps in a creative mode (pitch shifting / sound changing) that you would use them for - I watched the Spectral layers demos - all hugely impressive, but not sure why I would need it!! I have sound Forge but to be honest I don't use that much any more as I tend to do most thing directly in Sonar - so advice / thoughts / what you use these programs for greatly appreciated!

No I agree with you. And I downloaded a demo version? And the interface was heinous "at best...." It would be good to clean things up after the fact, or maybe if I want to cover something I could reach in and pull out a part to listen more closely to how it is performed. Other than that I'd say removing vocals for Karaoke or something. It's just pretty cool the way it works. Of course, their demo shows them removing a siren from a mix, which is considerably easier than removing a rhythm guitar.  

I've used spectral editing in the past to surgically remove low-frequency pops, coughs, chair squeaks, and various mystery noises and clicks. It's also useful as a diagnostic tool. You can see comb filtering, for example.