I'll describe what I THINK Tim is saying, and please correct me if I'm wrong.
Assume you've got an interface with a bunch of ins (we'll use the first 4 in this example - call them A, B, C, and D), and 4 outs (numbered 1-4).
You connect your studio monitors to outs 1 and 2 - so we call them your "main outs" - and set up your software to send the main mix to outputs on 1 and 2 on your hardware.
Meanwhile you've hooked up a synth with stereo output into inputs A and B (so you get both left and right channels) on your interface, and record a track. Hmmm, that track needs some reverb - not cheap reverb, but the sweet, vintage hardware reverb you have in your rack.
So, in your software, you send that the output of that synth track to outs 3 and 4 on your interface, and you run cables from those outputs to the inputs on the reverb unit. Then, you send the output from the reverb unit back to your interface inputs (C and D, sincer they're unused).
That's the type of thing you'd do with more outputs - its related to mixer buses, and they're description on the Tweakheadz site.
Hope that helps a little.