Gradation affects many properties of an aggregate, including bulk density, physical stability and permeability.  With careful selection of the gradation, it is possible to achieve high bulk density, high physical stability, and low permeability.  This is important because in pavement design, a workable, stable mix with resistance to water is important.  With an open gradation, the bulk density is relatively low, due to the lack of fine particles, the physical stability is moderate, and the permeability is quite high.  With a rich gradation, the bulk density will also be low, the physical stability is low, and the permeability is also low.  The gradation can be affected to achieve the desired properties for the particular engineering application.