Depth of field is traditionally understood in optics, and in particular photography, the area that comprises from the nearest and farthest point of our field that is acceptable in terms of sharpness, once formed its image in the same focus plane.
Application in Tilt Shift
Depth of field is used as a good resource for Tilt Shift, as it gives the feeling that large objects look like miniatures.
Application in Photomanipulation
Because it is inevitable that the cameras in reality have a given depth of field, and therefore, an area where there is sharpness and others where there are no, then if you want to make a realistic setting it is necessary to synchronize the depths of field (the sharp with the sharp and the blurry with the blurry).