The Power of Gray - The Photoshop: Professional Retouching and Compositing Tips, Tricks, and Techniques - page 77

Note
You can see in Figure 2.2 that the gray paper background isn't evenly lit. Having
dark and light areas and interesting shadows helps produce interesting pictures.
The Background
This blend mode compositing technique is used when adding solid backgrounds, such as
walls and doors, as opposed to something like an outdoor scene.
A key element of a successful composite is perspective; the angle at which the background
was photographed should match the angle at which the model was photographed. Getting
this right is actually easier than you might think; it just requires that you be consistent.