Commercial and Advertising photography is a bit different of other kinds of photography. Its main goal is to advertise and promote product, place or service. Advertising photography could be fashion, food or sport photography. The good advertising photography sells, that is why it is often most expensive type of photography. The other reason for that is the work involved.
For example when doing commercial for clothing company , a typical photo shoot would involve model, stylist, makeup and hair artist, as well as some times extensive pre- and post- production - scouting, retouching, shooting permits in the shoot is on public and so on.
Advertisement Photography requires not only perfect technical shooting but also creative thinking, collaboration and design. Although commercial photography could be expensive it is the type of photography that will generate you most profit.