It allows to integrate a 3D model in a real environment, making much more credible the set because part of the image is a real picture. In this kind of composition, the virtual models have to have the same illumination, shadows, etc than the real image to be fully integrated.
3D Integration renders are something between virtual and real. This is the best option when the object and the place where is located are equal important.
Choose the real picture and we will do the rest!