That depends. On my Monaco coach, when I turn on the Propane 12 volt WH switch, it lights up to let me know that the WH is powered.
When I switch to 120 volt, that switch lights up to let me know I am using 120 volt power to heat the water.
Since I can have either one or both switches turned on, depending on how quickly I want to heat my water, it is nice to have the lights in the switch work.
There may be some way to change just the bulb in my switches but when mine failed, I just replaced the entire switch, which was pretty easy since all of the wires just plugged in to the connectors.