Think of any object that is inviting to oxidation and you will probably be able to think of a protective coating for it. You wouldn't build a house without shingles on the roof, or drive your car in the rain without paint! So why leave your asphalt unprotected to the everyday elements such as rain and snow!
Asphalt is a very complex mixture of thousands of chemicals which begin to deteriorate after years of exposure to weather, salts, and chemicals such as gasoline. The first visual sign of deterioration is the change in the surface color from a black to brown and finally gray. These color changes take place as vital oils and chemicals are "baked out" by the sun. Ever looked at a parking lot and wondered where all the gravel on the surface came from. It was once part of the parking lot but as the bonding components were deteriorated the stones were loosened.
The next visual sign of deterioration is minor cracking. When not treated properly these cracks will continue to grow over time. As water penetrates through the asphalt and attacks the base, the load capacity of the asphalt surface may be dramatically reduced. Once the load capacity of the asphalt is reduced you may begin to experience problems such as rutting, sinking, or large alligator areas. If you wait until this point it may be too late to save the structure and a cap or completely new surface maybe necessary.