In Cities Skylines the Industry DLC added warehouses. The first one you can access I believe is the warehouse yard and as you progress through the game you get bigger options.
My question is, on the budget tab if you go to the industries sub-tab you can see that warehouses have an expense and income associated with them. I assume the expense is the cost of paying employees to work the warehouse.
But where does the income come from? Mine seems fixed at $0.00 for my single warehouse. I thought maybe it was tied to how much it exported stocked supplies but I don't think it is that. I have a huge amount of exports and the warehouse is full of them (generic goods) and it stays at $0.00 for income.