So 12v comes in from a laptop-style adapter (2.5mm positive tip) onto the PIANA PCB, from where it is immediately brought off on a 0.1" header, to feed power to Not So Teeny Tiny TV. The 12v is regulated down to 5v and pushed down into the Pi over the GPIO connector, the Pi itself regulates back to 3.3 and pushes that up to the PIANA PCB, from which the PIC and opto derive their power.
The advantages are about minimizing cabling and minimizing other expenditure. Just one wall-wart supply to give both 12v to NSTTTV and 5v to the PCB stack, a single power connector onto the PCB stack (or since it's a PCB sandwich, maybe it's a PCB snack? A Pi snack??), NSTTTV is powered from the PCB snack so minimal cables come in and out of the snack, composite goes up from the Pi to NSTTTV, and it can all be housed between another sandwich of laser-cut ply base and laser-cut acrylic top.
The intention is that the regulator will sit upright, rather than jutting crazily off the edge of the board. In case you were wondered about that crazy, tightrope-walking regulator up there. As you can see, the image is captured before the top and bottom polygon copper pours. The top surface is split into a 5v and 3v3 planes, the bottom surface is a ground plane. Note my now standard 'Defense against the Dark Arts' of a partial ground ring in blue and a partial power ring in red, to help the copper pour and avoid the dreaded 'Polygons may have fallen apart' ...
And as one of my STEM Ambassador tricks, I want to offer an after-school computing club project of 'let's spend 2 hours building some synthesizers'.