Mortise and tenon joints are used as a standard joint in several areas of the woodworking industry. It's probably used the most in timber framing, cabinetry, and furniture building.
The joint by itself is a very strong joint which is sometimes made stronger by driving wedges into the end of the tenon. This helps keep the joint tight and keeps it from pulling back apart. Sometimes this method is used in a blind hole where the tenon is driven into a blind mortise. When you drive the tenon in place the back of the mortise drives the wedge tighter.
Often the tenon is pinned in place in the mortise with a dowel or screw. This helps hold the joint together and, when used with dowels driven through the front of the joint, is also used as a decorative accent.
One extreme use of mortise and tenon joints is it's use in large wooden mallets. To hold these heads tight the tenon part of the handle would be driven through the head and out the backside several inches. After securing the tenon in place with wedges the head and handle would be soaked in water until they swelled up. This method created a very tight joint but would have to be soaked again when the head loosened up.