In Power BI or Power Pivot, you often need to create reports that go from the many side of the relationship to the one side. How can you do that without expanding your data model by adding more columns to your table? There is an easy trick you can use to solve this with a single DAX formula.
If you'd like to "see" how the DAX CALCULATE function works, then you need this tutorial. It will show you what's happening behind the scenes: How the CALCULATE function works and delivers results.