According to the official documentation:
Joins are a more static way to combine data. Joins must be defined between physical tables up front, before analysis, and can't be changed without impacting all sheets using that data source. Joined tables are always merged into a single table. As a result, sometimes joined data is missing unmatched values, or duplicates aggregated values.
Joins -
1) Are displayed with Venn diagram icons between physical tables
2) Require you to select join types and join clauses
3) Joined physical tables are merged into a single logical table with a fixed combination of data
4) May drop unmatched measure values
5) May duplicate aggregate values when fields are at different levels of detail
6) Support scenarios that require a single table of data, such as extract filters and aggregation