The dimensions of the fuse box simply describe its physical size. This is important for builders. When putting together a new design – or wiring a house – accommodations have to be made for the fuse box, as this will be a necessary piece of electrical equipment.
It’s also important to take into account whether or not the fuse box is intended as a surface mount fuse box or to be mounted internally, as this will affect what dimensions are appropriate.
In situations where a fuse boxes being placed as the central distribution box for an entire building, a fuse box with larger dimensions than necessary at the time is oftentimes chosen. This is quite simply to allow for the expansion of the fuse box in the future without having to replace it outright.