Head Pivot Layer

Hi everyone!

I was studying the rig of Heather, the WizardGirl and got curious why they put a head group named as FaceStuff (with all face layers, like eyes, eyebrows. mouths etc) attached to a layer named HEAD PIVOT.

This HEAD PIVOT is just a blue circle on neck/chin hight. Why they do this? Whats is de advantage? When is recommended to use this?

Thank you all and sorry the bad english.