Can't say as to Cubase, but in Reaper its useful anytime you want to be able to mix a subset of tracks relative to one another and then move them up and down as a group on one slider, or apply an effect to them as a group. Example, suppose I double up a rhythm guitar part, by playing it/recording it twice and then panning one track hard left and another hard right. I played a teeny bit louder on the second take which I've panned left, so have to notch that slider back just a bit so it sounds balanced. Now suppose I want to put a little bit of delay on them and a little bit of reverb. I could keep them as two separate tracks, put delay and reverb on each one and just make all the settings the same on those effects. Then when I mix the song, if the guitars are too loud, I move both sliders, remembering to keep the one notched back a little bit (and probably soloing them both momentarily to find that exact balance again).
OR . . . I can adjust the relative volume of the two tracks by notching one of the back a bit, then buss those two tracks over to a third track, and apply my effects only to that third track. Now, I only have to worry about one set of delay parameters and one set of reverb parameters, and if my R Guitar is too loud, I just move the one slider and I don't have to worry about upsetting the relative balance between the left part and the right part. The other perfect example is to buss the drums down for exactly the same reasons, either to a single track or to a few subsets... If you record a brass section, you can mix them relative to one another and then route them to a single slider so you can "Bring up the Brass, man!" or "Turn that sh*t down!" nice and easy. Multiple background vocals that need to stay the same relative to one another, but get the same effects and find their place in the mix as a group: sub-mix and buss them. Etc., etc., etc.
J