Speed.

You can periodically build a read-only cube that has pre-computed answers to any number that any pivot-table could read from that star schema.

Then, when the multinational supermarket CEO queries for his company's total profit by product line over the past five years, he gets an instant answer.

The other way, the CEO must wait for weeks to get that answer from the RDBMS - or the cube must be replicated the cube inside the RDBMS somehow using tons of materialized views.