I was fitted for one, and was told that the lower jaw is held forward by using the upper teeth as an anchor. Over time they can cause upper and lower teeth to move.
It was wildly uncomfortable and I gave up after three nights. Also didn't notice better sleep.
But the effect of jaw displacement is real. Every morning it takes a few hours to move the lower jaw slowly back. And I've visited a dentist in another country, who advised against it (and had apparently rarely seen it).
It's all local.