Concrete curb and gutter is generally installed at the perimeter of a road or parking lot. Not only does it define the edges of the lot, but it actually holds the pavement in place and serves as a “termination” to keep the pavement from expanding and deteriorating.
For example - an asphalt driveway does not have a termination edge, and you will usually notice that it breaks up along the edges first. The edge is the weakest point and as vehicles roll over it, it can push outward into the dirt area causing it to fail.
The part of the curb and gutter that you don´t see is approximately 8 inches thick, below grade, forming an extremely strong barrier.
The drainage patterns in most parking lots are engineered using the “gutter pans” of the curb and gutter as flow lines. Concrete can be placed much more precisely than asphalt can, giving even fairly flat lots the ability to drain. Curb and gutter is the first part of the parking lot to be installed and is done to a strict tolerance. The asphalt is usually placed so that it drains toward the curb and gutter, thus allowing the water to eventually flow to the catch basin or other exit point.