A figure-ground diagram clearly illustrates the relationship between mass and void, and it is a powerful tool that can be used to study the urban fabric of a city. A high ratio of building mass to open space allows for clearly articulated urban space and well defined connective elements to link those spaces. A low ratio of building mass to open space can lead to buildings disconnected from the urban fabric and can be indicative of surface elements such as parking lots.
A historical extension of this idea was utilized by Giambattista Nolli in his famous map of Rome in 1748. His map utilizes the same mass to void relationship utilized in the figure ground diagram; however, Nolli added an additional layer of information which included public spaces. Thus, when one views the Nolli map, not only is the void of street elements apparent against the mass of buildings, but the voids of public spaces such as churches are visible as well.
A nice little exercise involving figure ground diagrams is found here. The objective being to name American cities by their figure-ground diagram.