Why does a toaster oven need a circuit board?

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Tail light with a PCB? Sure, its probably an LED array that requires a driver/controller, regulator, PWM circuitry, heat sinks. etc. Suddenly makes sense now.

4 normal little incandescent bulbs, that's all.

PCB also probably tells the car's computer if the tail light is not working so the computer can inform the driver.

Probably. But my 98 Ranger tells me on the dash if a brake light is out, little light pops up when I hit the brakes. Doubting there's a circuit board in that tail light, though I suppose there may well be one somewhere else. If so I guess it's a shell game, you have one, only difference is where it's located..