Envelopes are used whenever you wish to make a volume (or pan) change on just a portion of a track.
Example: You have two singers singing two-part harmony. After balancing their respective volume levels you notice that on one phrase within the song, one of the singers got louder and the balance between the singers is thrown off. You put a clip volume envelope on the loud singer, and throw a couple of db drop at the start of the phrase, and raise it back to 0db gain at the end of the phrase.
Exzample 2: You have a lead guitar part (or sax, or whatever) that sits among the other instruments for most of the song - but you want to bring it forward in a couple of sections - maybe where it takes a solo. Put a volume envelope on the track and give it a few db of gain in those sections.
Bottom line, envelopes are simply fader automation. They raise or lower volume settings (or change panning) at certain times within the song. The degree of change and where they occur are settings you program into the envelope. IMHO they are the only way to automate, and you should never, ever look at the stupid Console view.
BTW, Sonar also has FX envelopes as one of it's neat new features. This allows you to change the amount of FX at particular spots in a song. It's the only recording software (as far as I know) that has this feature.
