It is very important to use an asphalt sealer on your driveway to prevent damage caused by sunlight, rain, and snow. A new, unsealed driveway will be fine for a few years, but neglecting to apply asphalt sealer to the surfaces of your property allows water to sneak beneath the porous surface. When the water freezes, heats up, or saturates the soil underneath the pavement, you will start to see cracks. Your only choice at that point would be relaying the asphalt.
Cracks in asphalt occur when the earth underneath the pavement shifts for some reason. This could be due to seasonal changes in weather and humidity. Asphalt is designed to resist this moisture penetration, since it has glue keeping the aggregate material together. Even still, asphalt is not impermeable to water damage, especially in a driveway.
When cold water sinks into or below the asphalt, it will freeze when temperatures drop. Freezing water expands, interfering with the stickiness of the glue. At some point, the water defrosts and shrinks, putting structural strain on the pavement. Pavement may buckle and make small fissures or larger cracks. It is important to use asphalt sealer on a regular basis, maybe every two years, as an added layer of waterproof protection.
Of course, asphalt sealer won't prevent cracks caused by earthquakes, but on the West Coast, there is another good reason to use asphalt sealer on your driveway. Slippery clay soil that is found in the west, as opposed to absorbent sandy soil, doesn't drain water well. Clay will keep moisture next to the boundary between soil and asphalt, increasing the chances that water will infiltrate the pavement.
Another reason you might consider it important to use asphalt sealer is to maintain the original, dark color of your driveway. The sealer will keep your asphalt black, rather than letting sun and traffic dull it to a faded grey. Asphalt sealer can also resist spills from car oil and other maintenance fluids that you don't want to stain your driveway.
If you owned a parking lot, you would have another reason to use asphalt sealer. The constant friction of tires will wear away the uppermost layer of asphalt. This layer has a lot of gummy petroleum material and not as many aggregate rocks. Once it wears away, the erosion of the parking lot will speed up. A thinner asphalt is even more likely to crack, buckle, or develop potholes.