So light is a good thing, right? Not always, no. If I ask you if you see light when you look down your ice shaft that has dripped water all over your refrigerator door, it's because warm air is melting the ice too quickly.
If your dryer won't turn on, but the light stays on when the door is shut, it's probably because the door switch is broken.
If the light in your microwave stays on when you close the door, the switch is probably broken.
But there are also good lights, like the one that says "Check lint screen." This is turned on before every load activation as a reminder.