OK, Let's just take a few seconds to try to imagine what it must be like for the lonely electrons that are sent out through the wire to light the brake lights.
Some electrons get to charge the battery and other fun stuff like that.
But no, Billy the electron must be the fool that lights the brake lights. He knows it's an important job but does this guy have to keep using his brakes?
But here we go again, out to the bulb filament. What happened, no way to complete my journey through this 25 year old socket, now what do I do? Oh, this is the only path for me - through the tail light filament which is in the same bulb with brake light filament. Whew, that was close. Now I get to light the entire running light circuit even thou the voltage is reduced by the brake light filament at least I get to go home to dinner tonight.