There are many circumstances in which you may want to add joins. Merge joins useful in joining new machine learning tables to the original source table, for instance. Or you may want to join a new Summarize or Date Range table to its source table.
Alternatively, you may have tables in the data source that you want joined in a single table in the new data model, to make navigation and querying easier for end users.
Join: add a join between 2 tables, combining columns from each table and adding the join to the database schema.
Union: create a union between 2 or more tables, combining rows from each table and adding the union to the database schema.
By default, the preview panel displays 50 rows, which may not be enough to preview joins. Use the Preview Size drop down in ribbon to show more rows in the preview panel.