What is the purpose of a half plate?
Certain keyboards are built such that a plate is required to build the board -- instead of being mounted into the bottom part of the case, the plate is often screwed into the top.
Plates provide rigidity which is frequently desirable with linear switches, but can be less desirable with tactile switches.Some of the nicest switches I've used were vintage PCB mounted MX browns. A polycarbonate half plate is about as close as you can get to PCB mount while still using a plate to mount the PCB.
It can also just be a different typing experience for linear switches if that's your jam and you just want to try something different.