You mean just basic subs? If so, they are used in live settings to send different mixes to different monitors. You could use them in recording to group tracks together (all drum tracks can be brought up or down the same amount, if you want). So, if you're talking about subgroups, that's what they do, they allow you to group tracks together. Like right now my mackie onyx is screwed up, so I'm waiting for my new one to come, so I have my subgroup 1 and 2 outs sent into my Omega so I can at least record some demo stuff while I wait. I end up with only a stereo track, but it's good enough.
I hope that's what you were talking about.
.....any help out there?