Some images advertise something, like an upcoming rock concert. Others exhibit politics, religion and Weltanschauung (philosophy, view of the world). Many stickers are just for poking fun and to provoke.
Street art also displays the freedom of expression. The artists present their philosophies, political believes, tastes, humor. Often you can see clusters of images produced by different artists which sometimes combine to a fascinating synthesis of art (Gesamtkunstwerk).
Thus the artists use the freedom of speech. In the U.S it is protected by First Amendment. But there might be conflicts with the interests of landlords, who might call the libertarian use of their properties as vandalism.
Anyway I am amused and entertained by street art I found in New York City, Cologne (Köln, Germany), Chicago, Dallas and other places. The world is a better place with street art.
Here you can see some tidbits of my photo collection I assembled in Manhattan - enjoy.
To be continued.