I bought mine a long time ago on Ebay from someone who had made copies of an old plaster model into foam. As the website says..."The Original Planes of the Head model is designed as a tool for learning in an effective and orderly progression how to draw and paint the human head. The model is applicable to both sexes and all races, with variations arising only in proportion. The left side shows the basic structure of the head's planes as seen in a rounded and or younger face. The right side shows a more complex structure characteristic of a thinner and or older face."
All heads have these planes; even if they are smoothed over by flesh and fatty tissue they are still there and visible to some degree. I use the model for informing flat/unclear reference photos and I often prop it up beside my monitor to look at the facial structure of a pose I am working on that may be in a difficult angle to draw from memory.
Because the model is white, you are easily able to see the directional changes in the surface, and thus the effects of light and reflective color on each part distinctly; to better grasp what is happening environmentally and affecting the head.
Looking at this picture of Arnie, we can see the planes pretty clearly. Notice in the middle shot how there is a slight blueness on the planes of the head facing the upper left. These subtle cues tell us about the things going on in the scene, like the sky color being reflected there, and thus it should be on all other surfaces of the scene. The redlines quickly show the divisions of the head as comparable from the white model, and we can more clearly observe distinct patches of value, color and form.
The new model for sale comes with a hole in the bottom so that it can be mounted on a tripod and held in place at any angle. The above picture shows an artist using the model to remind him of the distinct changes that should be happening over the surface of his portrait; telling him to make sure he has mixed unique and separate areas of value and color for the planes, however subtle they may appear.