A Cartogram is a representation of a dataset on a map. The representation is in such a way that the area on the map, which has more value will inflate. Irrespective of the boundaries and the areas with low values will shrink. It is like a choropleth map, but in a choropleth map, the size of boundaries remains the same.
Why do you need it?
We need a cartogram to have a direct clue, which region of the map is in progress according to data just by sight. It helps us to have an effective overview of the data and saving a lot of time.
What kind of data you can visualize with it?
If we have geographical-data with numeric values associated with the areas, then it could be beneficial. Assume you have a population count per country of Europe, then using Cartograms will inflate the size of the countries with a high population, and the countries with a low population will hide.