Here's my silly question of the day...

I was searching out a 39mm lens board for my newly acquired Beseler 23CII and came across a lens board with a pilot light. Please excuse my ignorance, but can somebody please explain what this is and why it makes a lens board so expensive?

The one I have for my 50mm lens (which came with the enlarger) is a simple piece of 4x4 aluminium with a 39mm hole in it.