It was made of stone or bronze and people somehow thought that it should last forever. But contemporary public art can come into the space, inhabit that space for a certain amount of time, broadcast aesthetic, political, social messages and then move away and make room for something that can inhabit the space later. It's not a monopolization of the space in perpetuity. One of the most important developments in contemporary public art has been the direct engagement of the community and the viewers of the piece with the artwork, so that viewers are no longer viewers, but participants, and they don't look at a piece, they take part in a piece. Artists are setting up situations for experience, rather than constructing didactic objects.
Public art is always on some level political because the use of public space is a political act.
In a traditional public art model what would happen is powerful elite groups would decide what message they wished to convey to the populace. It's a sort of political preaching. It's a politically motivated history writing. These images are often intentionally large. They want to inhabit the space, but they also want to inhabit the imaginations of the viewers to the point where the experience of that piece, of an image on the library, on the Bunker Hill Monument, will linger in memory and affect people's thinking about the space itself.