The need for dimensional modelling typically comes when:

* you want to build a SSAS cube on top of the data warehouse (as SSAS thinks in terms of facts and dimensions).

* business users need to create reports directly on top of the database. Dimensional modelling is generally easier to understand than a fully normalized model

* you want to store historical data efficiently

You don't need to use dimensional modelling, you can also use for example the Inmon methodology.