Compositing is useful when you do not have a final image to represent your requirements. Compositing is typically used for marketing, but not exclusively.
By combining photographs a new image is created that might not have been possible with the original photograph. Alternatly, a composite can also represent a product layout to be used as a graphic design for social media, web or advertising/promoting. Below are a few examples of composits used in marketing. Click on image to see full size.