Thanks Danny.
As to how/why it's used... Here's a novel - sorry about that, bro. :)
First, I must say the import functionality is very comprehensive - it can optionally import pretty much everything including Global Tracks (that is, your markers, tempo/tempo changes, meter/meter changes and all) plus midi, audio + associated channel settings, automation and all.
As I mentioned, for me it is handy because I do rely on versioning a lot. Not only versioning of the same arrangement, but I often have as much as 5 or 6 possible arrangements of the same song idea.
To illustrate that, say that I'm working on a funky mix and a rock mix, in parallel. The rock mix is going good but the chorus could do w/ a touch of funk and a hint of that wild midi clavinet part from the funk arrangement would be just perfect. That one hasn't been bounced to audio, but, if I did my homework, I probably saved that sweet sounding clavinet as a channel strip template and all I need to do is to load that strip and play the part again. Meh...
I don't know about you, but I can (and do) save lots of channel strip templates, the ones I need systematically seem to be the ones I overlooked, because I was just rehearsing a temporary part or groovin' away, you know... On top of that, I'm not sure I'll actually be able to top my previous performance and/or I don't have time to slip back into recording mode - or whatever lame reason I have... Quick solution: partial import feature.
A couple of clicks and that midi part and its settings are extracted from the funk project and merged into the current one.
Next, and that one is a matter of personal workflow, so it may or may not apply. I'm done recording and arranging, I've bounced everything and I am getting ready to mix. Personally, I like to start afresh and create a new project, with only the relevant audio, and maybe a couple of alternate tracks, just in case.
First use of the selective import feature - importing all the tempo and meter info as well as the markers. Obviously, you could accomplish the same thing by just deleting everything except that info in your project and saving it as a new project to rebuild your song. I just like the convenience of being able to re-create a project from scratch w/ only one or two clicks.
As I'm working on the mix, I suddenly realize that the song could be a bit more exciting if only the drums were just a tiny bit busier towards the end of the last verse, to build up a bit before the finale. Or that the snare sound just won't work as optimally as I expected, or that I'd like to keep only the left hand pattern on the piano part for the break and kill the melody.
Of course, I could re-open a previous version of the project, tweak there and re-export. Or I could use the selective import to add just what's needed in the current project, w/ the added benefit of being able to audition the changes against my current mix. Maybe muting all the melody wasn't such a great idea after all - I could use a couple of the more significant 16th notes here and there...
Another more obvious use, one which fortunately I haven't had to use - should a project ever get corrupted, selective import allows you to rebuild it entirely and to salvage a whole lot more than you would normally be able to retrieve in other DAWs - not just audio but markers, automation, midi, plug-ins, etc.
Overall, it just makes it so that, no matter if you prefer to bounce things early on for whatever reason and/or constantly tidy things up as you go, you can have all the advantage of file versioning w/o actually having to re-open those old versions.