The introduction of the weak relationships in Power BI composite models enables new data modeling techniques. However, not all of the many-to-many relationships can be managed by using weak relationship.

The “classical” many-to-many relationships in data warehouse is a design pattern requiring a bridge table, which is not required by a weak relationship in Power BI. The weak relationship can establish another type of many-to-many relationship that is different from the one commonly used in dimensional modeling, and it commonly solves a granularity issue in managing data coming from different data sources.

You can download slides, too.