I know that Brian May (Queen guitarist) would multitrack overdubbed guitar to get a symphonic guitar sound.
Used in a lot of today's numetal.
I went crazy with layering when I first started recording.
I do a bit of it on the internet with keyboard when I do new wave oriented post punk and on guitar solos and synths.
But I like to only gradually apply it to meat up the sound and mimic something on an instrument I added.
I am trying to master a basic song with my own type of artistic ideas and then add later.

But layering to me has always been represented as something to accentuate a basic recording, not for the sake of doing it.
I have heard a lot of ambient and noise acts that did nothing but hodgepodging sounds together.
Like The Cure and Kraftwrerk and a genre known as shoegazer.
But I can not explain about it as eloquently as these other posts have.
I am intrigued by the concept though.

Someone that is good enough with multitracking software can do stuff with layering that is pretty boundless.
I know that Audacity is leaps and bounds above what I had with having to deal with an outside playback device with the sound loss.

http://www.f3music.com/444