+1 to that back plate is mostly for look. Because the backside of GPU pcb is what you see in a windowed case, it really helps to make look neat and clean.
I have EK gtx 680 water blocks and back plates on two GTX 670 FTWs. There are some spots of direct contact (looks like vrm related part) on the back plate via thermal pads. I guess it will provide at least some passive cooling on these spots. But it definitely won't affect your GPU/VRAM temps.