Good site photos often have very clear foreground and background. At left there is a forest image that has good foreground and background. Using the magic wand tool in Photoshop, the background can be cut away from the image as demonstrated in the images in the lower left. If the picture can easily be divided into layers, it works really conveniently with Photoshop's concept of layers.

On the left at the top there is an image without much depth. This in itself makes the house look pretty bland. It may be easy to stick a model on the site, but the result could easily look disassociated from the site. In the image at right center, a high contrast between a tree and the existing makes cutting the two apart easy and lets something new fit in behind the tree. The blue house in the lower right has a fence obscuring it. This can be good as its now much easier to set something behind the fence without it looking too weird. The place where the ground and model interacts is hidden and can save a bit of hassle. But if you want to show the ground level of your intervention, the fence blocks it. And let's face it, that fence isn't very attractive.