All of the nasty things that end up on the bottom of someone's feet or tires will come into contact with these surfaces. While concrete is a strong compound, things like dirt, oil, grease, and chemicals that leak from cars will cause discoloration and damage eventually. Even walkways that only experience foot traffic will eventually become covered in many of these same substances. Some debris that gets stepped on or ran over by tires will basically become a permanent fixture on the surface of the concrete without a proper cleaning using high strength tools. The buildup of these things causes a dark or dirty appearance if the concrete has not been cleaned in a while. In some situations, a strong sealing compound can be used to minimize the effects and buildup of dirt and foot traffic.