Production Buildings are used to make the products you will sell. They do not make any income on their own, requiring distribution to sell the products they produce in order to make any money. Keep in mind that Production buildings don't exactly match distribution buildings in speed on a 1 to 1 basis. Some distributors can sell product much faster than its made, and some production can make a product much faster than its distributed. This means you'll have to find the right balance of production and distribution facilities to maximize profits.