Both taillights and brake lights are usually located inside the same globe. So, when both these lights are off, you can only guess which of them are taillights and which are brake lights. The purpose of taillights is to alert the drivers behind you of the presence of your vehicle. If there are no lights, other drivers will have to get closer than they should be before realising that they should slow down. But brake lights only light up when you push the brakes. When the lights turn on this will also make the already on tail lamps brighter.