If you’ve got dozens of vocal tracks, something as basic as “make the vocals louder” can become daunting. By busing all tracks of the same instrument to a single bus/group/VCA (the precise name depends on the DAW) you’ll have one fader that you can use to control the relative volume of that instrument as a whole. If you do this for all your instruments, suddenly even the largest mix becomes manageable. You may have hundreds of tracks, with individual hits, notes, sound effects, and background sounds per track, but if they’re all being submixed down to only a dozen groups, you’ll be able to adjust relative levels with ease.
When you submix all tracks of a given instrument to one bus, you don’t loose individual control over all of the tracks; all of the panning, send effects, and relative level adjustments of each individual track remain intact. So you can submix all your tracks consisting of one instrument to a bus for that instrument with confidence that you’ll still be able to mix each track to your liking.