When would you use the Data Model instead of formulas?

When you have several related tables, when volumes exceed what a worksheet can hold, when you need a correct distinct count, or when you need time intelligence such as year-to-date and prior-year comparisons.

The Data Model is the same VertiPaq engine as Power BI. Relationships replace lookup columns entirely, which is faster, smaller, and does not break when columns are inserted.

For a single flat table of moderate size, plain pivots and formulas are simpler and perfectly adequate.