Why use a hierarchical field in tableau?

A hierarchical field in tableau helps you to drill down data. It allows you to view your data in a more granular level.

When you connect to a data source, Tableau automatically separates date fields into hierarchies so you can easily break down the viz. For example, if you have a set of fields named Region, State, and County, you can create a hierarchy from these fields so that you can quickly drill down between levels in the viz.