It is designed to be simple and easy to learn, while exposing the power and flexibility of PowerPivot and SSAS tabular models.
Power BI also uses DAX for conditional formatting expressions and other dynamic properties of visual components.
DAX was developed by the SQL Server Analysis Services team at Microsoft as part of Project Gemini and released in 2009 with the first version of the PowerPivot for Excel 2010 Add-in.
Future versions of SSAS (both multidimensional & tabular models) will support DAX natively.
2016 marks a significant improvement with the rapid adoption of Microsoft Power BI and SQL Server 2016.