Since the earliest phases of 5G New Radio (NR), there’s been a push to disaggregate the BBU by breaking off functions beyond the Radio Unit (RU) into Distributed Units (DUs) and Centralized Units (CUs). The argument for disaggregation was flexibility, letting network operators decide how to locate these functions and maximize performance. Flexible hardware and software implementations allow scalable, cost-effective network deployments.
Because Disaggregation option will give you opportunity to choose which protocol reside within each function, and this is how the Open Ran is constructed.
DU contains some real time baseband processing unit, as It is near from radio unit as you see here, while CU is used mainly for non real time,
When the RAN is opened up horizontally, it could bring in a new range of low-cost radio players, and it gives mobile operators a choice to optimize deployment options for specific performance requirements at much better cost.