Maps are used to show a variable in relation to its geographic location. Multiple variables can be displayed on map, but the more variables that are added, the harder it is for the reader to understand the information.
An example would be this map showing the US 2016 per capita energy usage (energy usage per person) by state. Notice how the map includes a legend to explain what the colors mean, how each state is ladled, and the source of the data is listed. Additionally, notice how the map has a title that explains what the map is showing.