From their website:
Most collectors have a theme for their collections. Often this is a geographic area, usually the one in which they live or are otherwise familiar with, or something else such as the earliest of world maps after the discovery of the new world. Others specialize in a feature such as California shown as an island or other figments of imagination. Sometimes people collect examples of many cartographers or a single cartographer. Others might be interested in exploration or highly decorative maps or perhaps maps related to events such as the Civil War or industries such as railroads or even maps of places they lived or visited. As in other fields there are many directions in which to build a collection.