How may times have you ran out of breadboard space for additional circuits? I would say frequently especially if you build a lot of circuits. Maybe you have some extra breadboards but I but they are not neatly stacked unless you have one of those giant sized one.
Well, I used to have the same problem until I came up with the Breadboard Stacker. It holds an Arduino Uno and three broadboards in a neat stacked arrangement as shown.
I even made space to hold an LCD display, motor and servo; typical things that come in the kit.