ELI5: Why do broken automobile signal light bulbs stay lit but not blink when they're broken instead of just not coming on like a regular light bulb?

Many automobile bulbs have two filaments in them, a dim one (taillights) and a bright one (turn signal) and when one burns out, the other one will still function.

It makes it easier to diagnose if the issue is a broken bulb or a broken switch relay. Light is lit by default and doesn't switch means the switch is broken.

If the bulb is off by default, you can't tell if the bulb blew out or if the switch is broken. You'll take the time and money to swap out the bulb only to realize it was fine and the switch was the problem!