Some suppliers have several warehouses distributed around the globe so its important that when you import a product, the warehouses were that products is available, are activated in your settings.
If you try to sync a product that is available in a warehouse that is not matched in your settings, you will receive an error.
When you select the warehouse you want to work with, you are also able to rename them to be more flexible.
For instance, if you are selling in the USA market and you won't show when a product is coming from China, you can rename the warehouse China in "Outside USA".
Same thing if you are selling in Spain and you want to activate two warehouses China and Spain, you only need to rename the China warehouse in "Outside Spain" and leave the Sweden warehouse like that so the customer will see the Ship from or Warehouse (you can rename the attribute too, from Attribute tab) dropdown list filled with "Sweden" and "Ouside Spain"