Doors are tricky. The concept is relatively straight forward, but the execution is tougher. I understand that the shims are to hold space between the frame. I understand that the door frame is not necessarily square, so the gap going 'round jamb will not be uniform. All this gap stuff is important because it determines how the door opens & closes; if the door handle tongue lines up with the slot in the jamb; whether or not the door sticks when it closes. It all comes down to shims and gaps - they're what's responsible for the ease-of-use for doors.