If a Join adds columns, a Union adds lines.
Unions are useful when you have two (or more) tables with an identical structure that you want to combine to create a unique Data Source.
The typical use case is when you receive an Excel file with one sheet per year, and you need to compare those years. To do that, you need to combine those different sheets into a single Data Source. You can, of course, spend some time copying and pasting the data into a new, big Excel file, but with Tableau and Unions, you can combine them in a few clicks.
To create useful Unions, the different tables must contain the same column names. Otherwise, Tableau will not consider them identical and will create new columns.
Let's start with an example.