Having a mix bus lets you apply plugins to your entire mix without applying it to any sample mixes. This allows you to use the A B technique.
A sample mix can be placed inside of your DAW that skips the mix bus and goes directly to your speakers. This keeps the sample mix true.
If you apply the plugins to your master track, the plugins will also affect the sample track. Take a look at this video to see how mixing with a mix bus can help you improve your workflow and ultimately, lead to better mixes.