Hi Dieneba,

DAX functions enable you to create measures and calculated columns on data in Power Pivot/Data Model. To do this with a regular Excel function would require loading the data to the grid using a PivotTable or a CUBE function. i.e. if your data is in the data model then the best option is to use the DAX functions.

DAX functions are more efficient in a lot of cases because you can write a measure that aggregates the data in the data model without having to load the data to the grid, and then perform the calculation. If you were to perform the same calculation with Excel functions a lot of the time you would have to add some intermediary formulas at a row level of detail, and then aggregate those functions. 

Often it comes down to a need i.e. if your data is in the data model, then you have to use DAX. Your data might be in the data model because you need to model data from multiple tables, work with large data, or write measures that aren't possible with regular PivotTables.

I hope that clarifies things, but let me know if you have further questions.

Mynda