I have only used it a few times, but think it is a worthwhile feature. There is a tutorial in help topics which makes it pretty easy to use. Basically, it allows you to lay out a drum track, then decide where each individual note will send it's midi message. This lets you, for example, use your drum machine for a kick, your Roland for snare, your Korg for congas etc. etc. without using different tracks.
All in all, a pretty nice feature.
Take Care