The metal plate is an optional piece that provides a solid backing for the breadboard. The BB830 and BB400 have 3M adhesive tape on the back to attach them to a surface. The metal plate can be used if you are not attaching the breadboard to anything. Just peel off the paper tape and stick the breadboard to the metal plate. You can use the breadboard as-is, but the tape is a bit soft and spongy and the thin paper covering can be punctured. The metal plate provides a more solid, permanent backing.