Your visibility is reduced. Both YOUR ability to see what's ahead, and OTHERS' ability to see you. Especially if you are on a residential road with no yellow line, it is very easy to misjudge clearance between vehicles when one has a burned out headlight. It is actually illegal to operate a vehicle with a burnt-out light, and you can get a citation or a court notice if you're too lazy to fix it. Lots of people choose to turn on their brights when they have a burnt-out bulb, but THAT can get you a ticket too, for blinding oncoming traffic.