What is a star schema and why does Power BI prefer it?

A star schema is one fact table holding measurable events, surrounded by dimension tables holding the attributes you slice by, each joined one-to-many from dimension to fact.

Power BI's engine and DAX's filter propagation model are both designed around this shape. A flat table breaks relationship logic, prevents reusing a dimension across fact tables, and makes distinct counts expensive.

It also gives you a proper Date dimension, which time intelligence functions require.