Panelled doors wasn't a pretty extra but a clever solution to doors expanding with moisture and warping or jamming shut. Because wood expands and contracts in relation to the ambient air humidity (and also rain etc. if outdoors), the wood panels (not nailed or glued) could move freely inside the groves of the frame. This allowed tight doors that shut very close to the door frame, often mere millimeters or fractions of an inch, giving us ncreased privacy and better acoustics. And they look very charming.