I use it for songwriting. Very handy to change patches on a synth, record a couple measures, change again, etc. This was in one of the early "Friday Tip of the Week" tips. It avoids having to do lots of program changes, controller re-assignments, etc. I'll dig it out when I'm at a different computer and do an excerpt of the important parts.
Some of this is a question of philosophy. I regard virtual instruments as evanescent. Operating systems change, manufacturers go out of business, etc. Audio is forever...I can open up projects I created decades ago and if they're audio, I'm good.
Also as has been noted previously, moving large blocks of audio around is easy...MIDI, not so much. So by converting instrument sounds to audio, I can do arranging really easily.