Due to the way projector headlights are designed, they are capable of illuminating more road surface, at greater distances, than traditional reflector headlights. They project a more focused beam of light than reflector headlights, which means more light is cast directly ahead, where it's needed, and less spills out to the sides where it isn't.
Projector headlights consist of a headlight assembly with a replaceable bulb, just like reflector headlights. They also include a reflector component, but that's where the similarities end.
The overall design of projector headlights is based around the idea of focusing light with a specially-shaped reflector, then using a shutter to project it on the road with a beam pattern that is evenly-distributed and tightly-organized.