The joint you do not see

Wood moves with the seasons. A piece that is built tight in August will be loose in February unless the joints are meant to take it.

A through tenon with a wedge does exactly that. The wedge is driven in across the grain; as the wood dries, it pushes the tenon harder against the mortise, not looser.

It is slower than a screw and a bracket. It is also the reason a chair made in this way can be carried about a house for thirty years.