Treemaps are useful for representing hierarchical (tree-structured) data as a part-to-whole relationship. It shows data as a set of nested rectangles, and each branch of the tree is given a rectangle, which represents the amount of data it comprises. These can then be further divided into smaller rectangles that represent subbranches, based on its proportion to the whole.
We can show information via the color and size of the rectangles and find out patterns that would be difficult to spot in other ways. They make efficient use of the space and hence can display a lot of items in a single visualization simultaneously.