Combined Warehouses provide a "global" view of your sales and/or stock information - to generate Purchase Order recommendations based on total available stock against available against total relevant sales.
The Combined Warehouse calculates its own independent sales velocity, forecast, and replenishment recommendation based on data connected to it (sales and stock information), so it will be a separate result from summing the connected warehouses.
This feature is useful for cases where you can transfer stock between warehouses, provided stockouts aren't problematic*.
* For items with regular stockout issues - it's best to either a) generate orders based on the forecast/replenishment of the specific locations where issues are taking place, or to b) make adjustments to these items in the Combined Warehouse (eg. override past stockouts to match with stockouts from another warehouse, or apply forecast % increases as needed).
Past stockouts can have an impact on the calculation of sales velocity if/when the "Use Stockouts History" setting is enabled. If you are out of stock frequently in one location, the sales velocity and forecast will increase accordingly for that location. The assumption with a Combined Warehouse is that stock can be shared/transferred between locations, so if there is any positive stock it will not be considered a stockout day. Override the past stockouts in your Combined Warehouse if needed directly in Inventory Planner, or using a CSV import.
More information: What are stockouts? How do stockouts affect the forecast?
Lead time and days of stock can also be set for each warehouse. That means that replenishment recommendations are specific to lead time and days of stock settings for the combined warehouse.
More information: Setting up lead time and days of stock for your combined warehouse.
When creating a purchase order using a combined warehouse for forecasting and replenishment calculation purposes, you will need to select a different warehouse as the destination warehouse (or set a default destination warehouse in the combined warehouse configuration settings). The combined warehouse is a 'virtual warehouse' whereas the destination warehouse specifies a physical warehouse where inventory will be received.