My understanding is that OLAP is used to build over MS SSAS cubes and other things stored that way.
The old Designer (now UDT) is perfectly adequate for building universes over a data warehouse.
The other big advantage of a warehouse is to allow you to store data over time - historical snapshots and trends if you like. This means that as your warehouse matures, you can move from analysis into predictive analytics without any real difficulty by performing extrapolations of data trends.