Suppose to help with breathing.
Only people I've seen wearing them are "fitness influencers" and fitness "V-Loggers." They all wear them during their "morning routines" and their "full days of eating."
I'm guessing they all probably have "links in the description" for you to purchase your own. Mindset would be: "If my favorite super duper successful fitness star wears them, I should as well. And then I will be successful like them!"
Stay away from them. They are worthless. Don't make the influencers rich.