Metal plates have a variety of usage depending on the metal they were made from. Almost all can be used for making lanterns. Copper, bronze-type metals, iron and steel plates can be used to make brigadine and plate armor. Metal plates themselves can also be smithed into metal chains or metal scales directly. Some other smithing recipes can also use plates instead of ingots as the base ingredient, however most of these will result in a huge loss of invested metal in comparison, while making the process only slightly easier.
Copper plate is required for making the hopper and the distillery, while silver and gold plate can be applied in lanterns to increase their light level.
Metal plates can be "converted" into metal nuggets of the same metal type with the use of a chisel.
Metal plates can be placed on the ground, stacking up to 16 per block. They can be placed in storage containers as well.