I took mine off the cards ran cooler so no idea really how to answer why some say it run better with it, mine run better with out it .
looking at my msi r9 390, the metal that serves as ram heat sink is screwed to the back plate to better dissipate heat from the rams, maybe some cards have it for looks, and some have it for heat dissipation.
I have three RX 470 one of the RX 470 has one the other doesn't, the one with a case type back plate gets hotter then the one with out one i even checked the Vram on both with wattool no back plate one is cooler and the third RX 470 with no back plate runs all most as hot as the one with one . why i said I don't know or gonna try to explain why . by no back plate one has one but it has holes were it can short out and even be dmg by ESD it's runs the coolest of the three . the third one i can touch everything on back has no protection at all but is also is the lightest of the three it is very lite and the one in the case type back plate runs the hottest has no open holes on the back and is heavy. My R9 280 ran hotter with it before i watered cooled it i took it off , could have left it on but it seems to run cooler with no back plate .
watered cooled with a small fan from the kit blowing on the vram. Vram is even cooler .
I think the benefit of the back plate is to protect the PCB so that is does not bend so easily during handling.
