Alberto Ferrari explains why star schemas are so important to Power BI:
A common question among data modeling newbies is whether it is better to use a completely flattened data model with only one table, or to invest time in building a proper star schema (you can find a description of star schemas in Introduction to Data Modeling). As coined by Koen Verbeeck, the motto of a seasoned modeler should be “Star Schema all The Things!”
The goal is to demonstrate that a report using a flattened table returns inaccurate numbers, whereas using a star schema turns it into a sound analytical system.
Read on for the example.