Curbs may fulfill any or several of a number of functions. They separate the road from the roadside, discouraging drivers from parking or driving on sidewalks and lawns. They also provide structural support to the pavement edge. Curbs can be used to channel runoff water from rain, or melted snow and ice into storm drains. There is also an aesthetic aspect, in that curbs look formal and "finished".
Since curbs add to the cost of a road, they are generally limited to urban and suburban areas, and are rarely found in rural areas except where certain drainage conditions (such as mountains or culverts) make them necessary. Curbs are not universally used, however, even in urban settings (see living street)