Plate layers are for compositing as far as I know, to get realistic mattes and shadows onto the layer below. So if you wanted to create a scene where somebody was leaning against the Great Wall of China, you would take a shot of the subject leaning against a Blue or Green screen and edit that shot to overlay an image of the the Great Wall of China, then matte out the Blue or Green, what you should end up with is a person leaning against the wall with the shadow cast realistically, this gives the viewer a realistic perception. This is an easy task if you have a plugin that is created for this effect and can be used in such as AfterEffects and Photoshop, manually it can be a bit hit and miss but I would look up also look up Chroma keying to get more info.
