Tail lights function to let other motorists behind you know that you're in front of them. Thus, allowing them to not only notice that you're in front of them and they have to be careful. But also, motorists behind you are able to gauge how far they are from you based on their distance from your car's tail lights, and not get too close. Plus, those tail lights are always placed near or on the flanks/corners of a car. This further enables other motorists, especially in the dark, to roughly judge the size and shape of your car. Thus, they can get an insight into what they're trailing... A small car, or a huge truck. Just be wary of mistaking the tail lights with the brake lights, though. Unlike a tail light, the brake light only lights up brightly to tell others that you're braking.