What is the purpose of a curb?

This question has a number of answers, but here are some of them which explain why they are important.

1. Curbs help to control water runoff and, in conjunction with the crown in the roadway, direct rainwater to the side of the street where it will not impede traffic. 
2. Curbs, if they are high enough, help to keep the water off the sidewalks so that it is safer for pedestrians. 
3. Curbs help to keep vehicular traffic in the streets and off the sidewalks where they could injure a pedestrian.  If they are high enough (between 4 and 8 inches as recommended by PennDOT) they do, in fact help keep traffic in the streets and away from pedestrians.