They are a fad, much like the "perfect pushup" which you also see in gyms run by highly regarded trainers. A lot of the 'research' done by people who back up these training masks are done by people with PhDs (but not medical doctors) or "fitness celebrities."
Hell, the 'training mask' company had their "research" be written by a total clown (I think he had a PhD in Political Science). It was so much hokum that they actually had a "these claims have not been substantiated" all over their research page in little text.
Elevation masks do NOT simulate actual elevation, which is useful for endurance athletes. True elevation training increases red cell count (oxygen transport) among other things as your body adjusts to the hemoglobin transport curve.
Despite being called an 'elevation trainer' the mask is not. It is a hypoxic trainer.
What the masks will do is restrict airflow, thus making your lungs work harder. The actual effect on VO2 max or anything is basically nil, but it does help strengthen your diaphragm which WILL improve your ability to suck air. They will enforce and reinforce the 'breathing' muscles (scalenes, intercostals, abdominals, etc.) which allows an athlete to maintain a higher breathing frequency for a longer amount of time. Unfortunately it is also likely to increase skeletal muscle fatigue because the body is prioritizing the breathing muscles.
Net, it's a fad.