Very simple maps may be well-defined with just one symbol per layer, but this is not the case for the vast majority of maps you can find, nor for what you will create with your GeoServer. To fully express the meaning of features, you need to apply a symbology that can make it easy to recognize different real features on a map. Think of the road layer containing North America's roads--a map where interstates have a different symbol to intrastate roads, or where the federal road is much more readable. Countries symbolized according to their GDP can be mapped as the richest area of the world.
There are many different kinds of thematic maps. One of the most common is the choropleth map; we talked about it in Chapter 1, GIS Fundamentals.
Of course, SLD can be used to build choropleth maps; you just have to define a classification rule and a symbol for each class.