A common optimization in 3D graphics is a technique known as back-face culling. The idea is to treat polygons as essentially one-sided entities. A front facing polygon needs to be rendered but a back-facing polygon can be eliminated.
Consider the dinosaur model. When the model is rendered, the back side of the dinosaur will not be visible. If the direction each polygon ``faced'' was known, OpenGL could simply eliminate approximately half of the polygons (the back-facing ones) without ever rendering them.
Notice the calls to glFrontFace when each solid display list is created in extrudeSolidFromPolygon. The argument to the call is either GL_CW or GL_CCW meaning clock-wise and counter-clockwise. If the vertices for a polygon are listed in counter-clockwise order and glFrontFace is set to GL_CCW, then the generated polygon is considered front facing. The static data specifying the vertices of the complex polygons is listed in counter-clockwise order. To make the quads in the quad strip face outwards, glFrontFace(GL_CW) is specified. The same mode ensures the far side faces outward. But glFrontFace(GL_CCW) is needed to make sure the front of the other side faces outward (logically it needs to be reversed from the opposite side since the vertices were laid out counter-clockwise for both sides since they are from the same display list).
When the static OpenGL state is set up, glEnable(GL_CULL_FACE) is used to enable back-face culling. As with all modes enabled and disabled using glEnable and glDisable, it is disabled by default. Actually OpenGL is not limited to back-face culling. The glCullFace routine can be used to specify either the back or the front should be culled when face culling is enabled.
When you are developing your 3D program, it is often helpful to disable back-face culling. That way both sides of every polygon will be rendered. Then once you have your scene correctly rendering, you can go back and optimize your program to properly use back-face culling.
Do not be left with the misconception that enabling or disabling back-face culling (or any other OpenGL feature) must be done for the duration of the scene or program. You can enable and disable back-face culling at will. It is possible to draw part of your scene with back-face culling enabled, and then disable it, only to later re-enable culling but this time for front faces.