This allows songs/templates with virtual instruments and effects inserted to open lightening fast by loading only the instruments and effects in use. So you could load, say a huge orchestral template with all instruments inserted, but the song will open just as fast as a blank one. Then as you go to record on a particular track, that instrument "activates".
Logic Pro X introduced this a while back. It was one of the tough things to leave behind when I switched to S1. Having used S1 for a little while now, I'm sure Presonus would make it even more efficient.