Photography has been used for many things since its creation in the 1830s. Among these have been recording history, taking portraits, discovering new worlds and selling products. This last function, while it maybe not as noble as the others, has been used widely for most of the 20th century, and in the 21st century, despite a move away from print photography, illustration photos are often used to represent and sell products.
Illustration photography was first used in a magazines, newsletters and on TV to show readers and viewers exactly what an advertisement was talking about. For example, when a store wanted to sell select clothing, the promoters could either write the description and/or have a wood-cut drawing done for reproduction in newspapers and magazines. Once the idea of illustrating the sales pitch with a photograph caught on, the customer could actually see the clothing without having to go in to the store.
Catalog producers, such as Sears & Roebuck, wanted to get the most information they could about each of their products into the hands of potential buyers. The illustrations, at first, were drawings, but popularity soon turned toward photos. Following this market growth, such companies either employed full-time photographers or hired independent ones to design and create photo layouts that showed products in their best light. Elaborate shoots were executed, with men, women and children models posing in clothing on the beach, at school or in the business world to sell the items.
Another common use of illustration photography is the how-to book or article. If the writer is explaining how to take apart and reassemble a Macintosh Powerbook, for example, step-by-step photos showing each instruction help the reader to visualize what she needs to do to complete the task. The old adage "A picture is worth a thousand words" works particularly well in this case, since the user theoretically will be following the steps outlined and photographed.
Photographers also can use their photos to illustrate the items and routes at historic sites or museums. Brochures and booklets guide visitors to a historic site by giving a photo for each destination, along with a caption that tells visitors what they are looking at. For museums, photographs illustrating the exhibits are often used for self-guided tours.
In essence, illustration photography is just what it sounds like: using photos to illustrate an action, story or item that is presented better with some form of visualization. It is a function photos generally fulfill better than alternative illustrative methods, such as drawings or paintings.