Street posters play an important role in participatory civic life, according to this article from re:place magazine.
"To some, posters might be seen as little more than crass, obtrusive clutter. Yet for others they are essential to the lifeblood and culture of a place. Thus, it is important to understand the forces and attitudes that lie behind street posters and the things that allow them to keep popping up even in places they are not supposed to. Their existence is tenuous, but the role of posters in public life should not be overlooked. The intent of this article is to draw an understanding of street postering in Vancouver. As such there is a need to explore the significance of posters, regulations, and the intriguing – if not confusing – culture of postering in the city."
"As a medium, street posters are profound in their accessibility, directness and simplicity. From an objective point of view, street posters embody a civic culture that is messy, subversive, resourceful, and sometimes even deviant. But from another perspective, street posters also reflect something deeper: the creativity, entrepreneurship, passion and political ideals of communities. If posters were not an available medium it is probably fair to say that it would mean the loss of an essential component of participatory civic life. It is also not unrealistic to suggest that a city without street posters would be a much less interesting place not only to look at, but to live in as well."