It also keeps your muscles relaxed and arguably allows smoother, snappier straight punches.
Not a coincidence most of the best jabbers in history held their lead hand low(Holmes, Clay, Foster, etc)
Well one fluid longer movement is going to gain and share momentum better so I am buying that it makes them snappier. If you think about how a straight punch works having your hands high slows down the process and makes the motion jerky so you are either going to end up throwing from your elbow or being significantly slower from start to finish.
In other words rather than the entirety of your arm in one motion with equal momentum you are going to end up starting your movement from your elbow. As a result your hand to your elbow and elbow to shoulder are not only moving separately but at different speeds.