Volute
What's it for?
Volute
A volute is a raised, curved area carved into the back of an instrument neck behind the nut, where there is a transition from the peghead to the shaft of the neck:
Sometimes this volute is called a "handstop." Some people suggest that it's there to reinforce the area behind the truss rod adjusting nut pocket, but relatively few instruments with truss rods actually have volutes.
I assume it's more of a style thing, harking back to some earlier instruments. . .