The backplate is what keeps the card in the case, it has the screw mounts or where the screwless grips grab on to.
Do not use a video card if it's not mounted in the case like that, anything over a very cheap light card will not be held in place by the PCIe clip.
It also keeps dust out of the case.
See bellow, if you remove the backplate how are you going to mount the card in the case solidly? The screw connections are on top of the backplate.