SAP BPC is a reliable workhorse in Planning and consolidation space. The BPC we know and rely on has proven capabilities like business process centric tool, used and managed by business users, familiar excel interface, comprehensive script logic capabilities, robust security interface, ability to publish book and distribute offline reports and many others.
SAP BPC has served very well to corporate finance groups over the years and help them to manage corporate and LOB planning processes. The corporate team rely on salespeople spread across the geographies for sales related inputs, the cost center managers provide their own budget and forecast data to corporate teams and so are other functions.
The BPC excel interface help these LOB teams to integrate their own excel sheets with more granular data with corporate planning templates, but this is not an ideal way of planning in this cloud centric world.

The SAP Analytics cloud with its native two way integration with SAP BPC allows planning teams to provide zero footprint data input or collection mechanism to line of business users. The LOB teams can create their own supplementary planning models and lower level calculations in SAC and sync it with SAP BPC
Key Areas where SAC can enhance BPC experience
SAP BPC is great in handling complex user requirements and formats using EPM add-in but it does not provide a ready to use dashboarding tools or ad hoc analysis tools
SAP BPC provides unified planning model and present the data in easy to use formats but there are no capabilities to carry out predictive analysis
SAC allows user to easily create dashboards, data grids, do ad hoc analysis and predictive modeling by simply importing the ready to use BPC model in SAC.

The typical use cases for SAC and BPC
SAC as reporting and Dashboarding for BPC: This is the low hanging fruit in SAC- BPC landscape. A simple read only BPC model can be created in SAC for reporting and dashboard creation.
SAC as a data input interface for BPC: SAC can be used as a data input tools for departmental teams with BPC acting as a main planning model
SAC as a predictive tool for BPC: This is a specific use case where BPC BPF step can open an SAC story with prepopulated BPC data and user can run the predictive analytics feature to generate a forecast and submit that back to BPC for further optimization.
SAC as LOB Planning tool with BPC for Corporate Planning: This is the most desirable use case where SAC is used to do LOB planning and data get synced back with corporate BPC environment. The reporting can be done from both BPC and SAC
If one consider the complimentary capabilities of SAC and BPC, This extended planning solutions provide robust on premises planning solution extended with web based planning, reporting, visualization and predictive capabilities with a native two way sync of transaction data and master data.
Happy Planning!! Image source : https://blogs.sap.com/2019/04/15/hybrid-planning-scenarios-built-by-sap-analytics-cloud-for-planning-and-sap-business-planning-and-consolidation-webcast-recap/