When a timber frame load exerts outward thrust on this through tenon with a wedge, the force is well resisted and kept in check. The wedge draws the joint tighter, adding extra strength that you could not get with just pegs. The end of the tenon is rounded in this example, which is called an embellishment. The tenon is extended well beyond the wedge to resist the loads that are being place on it; this is often referred to as relish.
This type of through tenon is commonly found in anchor beams in Dutch style barns, which have been part of the New England landscape for hundreds of years. Therefore, it is often called a dutch barn style through tenon. The through tenon strengthens and secures the anchor beams, resisting the outward thrust.