A single layer is not limited to one of each symbolizer type. For example, multiple semi-transparent line symbolizers can be assigned to a polygon to achieve a soft glow or shadow effect. Multiple text symbolizers can be assigned to the same point with different offsets to label it with more than one field.
Normally when you assign a style to a layer, the style applies to a default symbolizer that is created. In the following example, the second rule overrides the first one because they both apply to the default symbolizer.