Cross sections are important to present what parts do we have in a road, for example. It simplifies the things for us and make the more visual. So, a road is consisted by a roadway which has surface course, solid lines and broken lines. But, besides its has a base course, sub base, base. Next, under it it has a subgrade, an embankment and a bed. Finally, all of this is laid out in the earth foundation.
Cross Section and Elements of a Road
See the other sections on the pictures below. There you can see several other sections that show you different aspects and different parts of the road.