So why? what are the benefits of using a bus instead of having all the tracks separate? Well, the main benefit is that it allows what I will call "macro" volume changes. Let's say you have a 10-piece drum kit that is all nicely mixed together, but you realise it is all a bit quiet compared to the rest of the track, it is easier to raise one fader than ten, especially if you want to keep the relative volumes the same. By having the channels grouped into a bus, they will maintain their mix levels with each other regardless of the volume changes to the group as a whole.
The image above shows a project I'd been messing with recently in Reaper. You will see that all the yellow tracks are indented (by the record buttons) below the orange track. This means that they are all a group within the orange track. We can call that our synth bus.
Using a track grouping also breaks down the tasks of mixing into smaller chunks. Instead of having to mix every track, with every other track, you can put the drums in a group and focus on them. Once you know that is nailed, you may want to move onto the bass, before moving onto the guitar bus.
When you know the drums sound good together, and you know the guitars, for example, sound good together, you can start mixing those two groups into each other, the task becomes a lot simpler than trying to mix each guitar track into the drum group separately.
Of course, changing the volume of several instruments at once isn't the only benefit of using busses, often it is a great way to add an FX plugin or chain to the group. If you were going to add it to all the tracks anyway, you will use less CPU by placing it on the bus. The other major reason for using busses with effects is that it is less messy as seen below from a previous post:
"I should note that I used to write songs where each instrument had a different reverb and they never sounded natural. It is often-times more pleasing to the ear if all instruments sound like they are playing in the same room. By placing the reverb on bus groupings, it can sometimes sound significantly better than by tweaking it for each track. Exceptions to this may be vocals, especially lead, and the guitar solo above, which may want different levels to stand out a little bit."
Reverb, in it's nature, messes up and blurs the sound. By adding a reverb on every track, there is a lot of added noise and blur (not to mention lot's of wasted processing power) since you really only want one reverb setting, you may as well apply it to a bus, saving time and unnecessary cleaning of the resulting sound.