My use of it comes from early 2010s era paleo-food community observations about hunter gatherers having less crowded jaws than agriculturalist and modern peoples, hypothesized to be due to both micronutrient intake and also eating tougher foods. Cramped jaws and facial structures lead to both malocclusion and also nasal breathing issues, and mouth breathing also reinforces those pathological facial changes. Granted, this stuff is from sources that I've heard wildly varying opinions on (Weston A Price; Mike Mew). Jaw and facial structure continues to change into adulthood. I needed extensive orthodontics in my youth and have struggled with sleep apnea in adulthood. This broad set of hypotheses made sense to me, and chewing mastic gum was the easiest way to get a full dose of "chew tougher food" in my life. I try to snack on raw carrots too.
Speaking to attractiveness, there's an uncomfortable photo of identical twins in one of the Weston Price books, one given orthodontic treatment that widened their facial structure and one was not. The one with the widened facial structure was more attractive. The difference wasn't gigantic, but it wasn't small either.