Put a Volume envelope on this and you can automate output levels. The fader won't actually move but the effect is there and can be monitored with the Output Meter bars.
Put the envelope on master if you want to be able to dip or boost all tracks at once, or on individual tracks for more localized control. It depends on your needs.
The real-world comparison would be the main bus fader vs. channel faders on a mixer; the ability for Vegas to create extra busses and map tracks to them is similar to the submix faders on many mixers.
To give a real world analogy.
If you had four mics on the drum kit you use each mics fader to set the balance for the cymbals, floor tom etc. Then you feed the mix of all those channels (tracks) into their own bus with a fader. Now if the drum kit is too loud / soft overall you can turn it up/down with one fader.