Why YSK: Your signals alert other drivers as to what you’re doing; a signal bulb costs a few bucks and is usually a quick and easy repair to do yourself (consult YouTube); and any place that regulates motor vehicles probably requires you to have working turn signals. So knowing when and how to replace a burned out signal bulb can save you an interaction with law enforcement.
Adding: You can diagnose which bulb is out by turning on your hazard lights and checking all four corners of your car. It’ll be the one not flashing.
This is also probably a good time to check your brake lights. Put something heavy on the pedal or have a friend hold it down and check that all three brake lights illuminate. Replacing a burned out brake light is also usually pretty cheap, quick, and easy.
I didn’t say it necessitates it, it’s just something I might do when I’m bored on a long journey. It’s never something I’ve done out of anger, if anything slowing down and speeding up again unnecessarily would increase my annoyance. It’s a mild bit of fun, though.
Laws change over time. Where I live, it never used to be explicitly illegal until about 10 years ago - before then it was just “against the Highway Code”. All advanced driving courses I’m aware of would teach you to move over, though.