Almost every design decision on a vanity comes back to light. You are looking at your own face at close range, and whether that is comfortable depends less on the mirror than on what surrounds it.
A glass floating on open posts lets the room show through behind it, and the eye keeps adjusting between the bright reflection and whatever is beyond. Setting the oval into a solid shaped backboard removes that competition - the reflection sits against an even dark field, and the face in it reads clearly without the room pulling at your attention. Cabinetmakers arrived at this by experience long before anyone described it as contrast control.
The scrolled sides of the backboard exist for the same practical reason as the panel itself. A plain rectangle of solid teak that size would look like a headboard, so our carvers sweep the shoulders upward and pierce a small opening in each one. The panel keeps its calming solidity, but the silhouette lifts and lightens.
Each backboard is cut and shaped by hand from a single wide board, the oval opening cut before the outer profile so the frame width stays even the whole way round.