This can be anything made of wood! Because wood is a porous surface and degrades over time, it can get very dirty. A good cleaning with a power sprayer can completely change the look of an older wood surface- fences, decks, outdoor furniture, and siding.
With enough time pristine hardscapes can collect a lot dust that gets blown around throughout the year. Mix that wilth rain and snowfall, and that dust gets caked onto any hard surface. A good power washing can renew your hardscapes!