When shipping an individual order that has some products allocated to more than one warehouse it can cause confusion at the packing station. Sometimes packers who are in a rush accidentally ship the items for the wrong warehouse, or they have to slow down and double check every order before shipping
Now when you go to ship an order the users Default Warehouse for that order will load first. Meaning your packers can work fast with the confidence that they are shipping the right part of the order.
To set the default warehouse for your users go to Settings>Users. Edit the user and choose the warehouse from the Default Warehouse option.
For more information on setting Default warehouses check out This Guide