Reading this thread may give you some ideas. Using the search icon at the top of any forum page can dredge up all sorts of stuff from previous threads.

If you're brave, (and wealthy), you can get into some interesting experiments that may, or may not, expand the collective knowlege in the field of speaker enclosures.

To reply to your original question: The depth of the cabinet is just one dimension, along with the height and width, that determines the volume of the cab. The volume is a major factor in determining the resonance of the cabinet, which in turn affects how it reacts to the speaker.