Hi,
Customers using SAP Customer Activity Repository don't have to use BW if they don't have an existing BW system. Using Hana as the underlying database provides OLAP and OLTP capabilities within the same product, so there's no need for customers to use a dedicated OLAP product such as BW. The data is directly available in the same Hana tables without the need to an additional data store (BW Info objects, DSOs).
SAP Customer Activity Repository also provides a set of Hana calculation views, similar to BW Cubes, that provide real-time access to data, these views can be used by BW in case the customer have existing BW objects or can be directly consumed by other clients.
In the end, the main idea is to provide OLAP and OLTP capabilities within the same platform.
--Ibrahim