The arrangement of elements within the image to form an internal frame around the subject, a frame within a frame, is a predictably successful technique - so much so that it's less-than-well-considered use has established it as a cliché. However, it is nonetheless a powerful and tasteful device in the right hands and creative eye. 'Frames within a frame' works because it does two things: it enhances perspective, and it introduces movement.
Depth of the visual field is emphasised because another intermediary plane is created and pointed out. There are a minimum, therefore, of three planes: the viewfinder/camera's outermost; the internal frame's, intermediate; and the subject's, innermost. The viewer's eye flits dynamically between the boundaries of the camera frame to those of the internal frame; and the visual momentum causes the eye to move deeper into the internal frame to the subject.
Depending on the shape or design of the internal frame, the viewer's eye can be made to follow along the periphery of the internal frame before being drawn into its interior.