When coming to building something, some materials are more vulnerable to water and can wear away easier. Some can keep the water in, so depending on the material, the water quality can be better sustained.
As compared to gravel, asphalt is an impervious material, thus it allows water to flow over its top without infiltration. In this process the impurities present in the water do not get removed but is gathered and carried away till the maximum concentration point is reached and then removed at later stages. While if there would have been a permeable material like gravel etc, the impurities in water is not discarded but is retained due to infiltration at unwanted locations.
