To add depth to an image – especially when the item acting as the frame is not in focus.
To bring contrasting elements into the photo without detracting from the subject.
To create structure.
Whether it be a very literal frame (that went through a popular “prop” phase), environmental, or structural framing, framing with light or lack of light, they all work well to bring the viewer's eye to your subject. Framing is actually defined by wikipedia as 'a technique used to bring focus to a subject'.