What purpose does a graphics card backplate serve?

back-plates serve 2 purposes:
1. prevent the PCB from warping or bending. high end gpus have heavy heatsinks, and can bend the PCB while sitting in the case.
2. The back-plate can help dissipate some heat from the bottom of the card. This is very useful if the card has ram chips on the back.
Manufacturers have stopped including them mainly because they save 5 bucks. EVGA still offers them though. unfortunate that companies started skimping out on design. of course, I made my own out of plastic for my amd hd 7850, since my computer moves around a lot.

in addition, many of the better oem coolers screw into the pcb, eliminating ther primary need for a backplate. these coolers are rather rare. look at the powercolor amd hd 7850