Why do organisations normally charge overheads to products in addition to the direct costs involved in making the products?

To make a profit of course, simple business

To cover ALL the costs of making a product. For example - A used car lot does not include the electric bill as a part of costs of goods sold, it is an operating expense (overhead - not specifically tied to any product, but necessary to make all products). However if the used car dealer only made a profit off of each car he sold and did not recoup the electric bills, the electric company would shut off the service and he would be out of business.