As for “improved visibility”: The goal of a flare is not to make everyone see you and to let you see what you are doing. The goal is to say “Hey, something is over here. Be careful”.

Right. For which visibility is important.

You do realize that your tail lights/brake lights are more or less the exact same LEDs, right?

Yup. Which people proceed to slam into in those many-car pileups in the fog when they can't see far-enough away that something is wrong despite people having their hazard blinkers on, and nicely illustrates why you would want to have magnesium flares.