The ceraminc granules are there for two reasons. The primary reason is to protect the shingles from the sun. The sun's UV rays are very damaging to asphalt and cause it to deteriorate prematurely. This is one of the same reasons that gravel is used on built-up roofs. The second and more obvious reason for the granules is aesthetics.
Asphalt shingles are available in a wide variety of colors to match almost any facade or landscape.
By far, the more popular shingles are the glass fiber ones. This may be attributed to the fact that they are cheaper and easier to manufacturer than organic shingles making them more cost effective to the homeowner, or it may be that they are easier to work with, or they may simply be a personal preference of the roofing contractor. The lifespan of asphalt shingles depends highly upon the environment. Shingles in cooler climates such as the northern United States seem to last longer than those installed in the warmer climates. Studies have shown that the average lifespan for a 20 year shingle in Phoenix, Arizona is around 14 years.