You mean why would you have one last stereo Aux fader before the master fader?

You don't have to, in fact, you could have nothing but audio files and everything would be fine.

But there are various reasons to have a final stereo Aux before the master. For one thing, the master has no sends on it. For another, there's no panning on the master fader, and that's one way to check mono instead of putting a mono master fader at the end.