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.
Specifics. How is what I’ve described being an asshole?
In particular, as it apparently isn’t immediately obvious, I’m talking about a 3 lane road. With a driver sat in the middle, you overtake them, pull ahead, then move over to their lane, then move over again to the empty inside lane (where they should be) and then slow down with them still in the middle lane. You’re not brake checking them or anything, you’re demonstrating more care and attention than they are, with them sat in the middle lane without overtaking and generally obstructing traffic.
What’s wrong with that? It’s less obtrusive than honking your horn, which arguably would be appropriate when someone’s sat in the middle lane without overtaking anything.