Asthetics...
Dirt, smog and weather all contribute to a gradual build-up of residue which changes the appearance of building surfaces. Power-washing removes that residue and restores the surface to the original intention contributing to the total uniformity of a location.

 
Preventative...
An accumulation of surface residue increases exposed areas to water retention. Harmful pollutants and other foreign substances interact with the environment and surface erosion begins to occur. Deterioration of surface areas can also occur simply from toxins within the environment.