Easy question: What's the aluminum plate that comes with solderless breadboards for? It comes with the board, but it's not attached, which suggests to me that is it sometimes used and sometimes not.

 

Breadboard terminals are often held in place by a vinyl sticker, kind of like a large piece of electrical tape. This insulative layer also prevents terminals from being shorted together if the breadboard is placed on a metal surface. One of the worst things you can do to a breadboard is to press it against the back of a through-hole PCB as the clipped lead ends can pierce the vinyl. The aluminum plate or some other rigid backing is useful to prevent the breadboard terminals from being pushed out the back of the breadboard since only the strength of the adhesive on the vinyl would otherwise do so. You will often see breadboard modules attached in multiples along with bus strips and binding posts on a metal plate with rubber feet.