Why diagrammatic maps?

Lets start with the definition of some key terms.
Map: the dictionary defines a map as a representation, usually on a flat surface, of the features of an area of the earth, showing it in its respective forms, sizes, and relationships according to some convention of representation. However, on a closer inspection of the history of maps, it can be seen that maps not only can be used to represent information related to geographical issues (i.e. areas of the earth). Maps are no longer only about physical space.

Content: by content is meant something that is to be expressed through some medium. As it can be seen in the images below, maps have evolved from geographic contents to a long list of non-geographic narratives.

Social maps. Football Diaspora, 2005. Map showing the global spread of the football game since its conception in England in 1863. (Abrams, J.; Hall, P. Else/Where: Mapping. University of Minnesota, 2006)

Political maps. Mapping European Politics on Migration (artistic project in the context of Transit Migration and as contribution to the exhibition “Projekt Migration”). Germany - Switzerland / http://www.transitmigration.org/migmap/home_map3.html

Scientific maps. Mapping topics and topic bursts in PNAS. Visualization of keywords appearing in PNAS between 1982 and 2001 (April 2004) / http://www.pnas.org/content/101/suppl.1

What is more, not only the kind of content changed, the amount of information represented in a map did it as well. Maps have become complex pieces of information that represent connections between data, categories of data, functions of data, evolution of data, statistics of data, etc.

Geographic: this term is usually related to describe in detail natural features, population, industries, etc., of a region or regions. The majority of maps before the 30s were geographic maps.

Diagrammatic: most dictionaries take this to mean a geometric representation, employing simple rectilinear or curvilinear lines or figures that help to clarify the relationship between the parts of a whole. At the beginning of the 20th century this was the term mostly used to describe the new layout that maps were adopting.
A diagrammatic language, as John Walker explains, employs a variety of means to encode information and every single element inside the map has a specific function.

Maps, where geographical accuracy has been abandoned in favour of clarity and has been replaced by a diagrammatic language, are called diagrammatic maps.