The underground cities of the Daraz are always built around a subterranean hot springs pool (of which there are many throughout the Valley). While all Daraz engage in similar bathing rituals, it’s especially important for the Harcos, the warrior/hunter caste.
Their body armor is made if very thick skin and/or keratin, and requires a great deal of maintenance to keep clean. Without regular exfoliation, they run the risk of skin infections, particularly down in the joints where the plates meet. Harcos organize into quads (groups of anywhere from 2 to 5 individuals, though the most typical number is 4, hence “quad”), and quadmates are responsible for grooming each other.
The act of being groomed releases endorphins and tends to make the one being groomed a little (or a lot) sleepy. As such, it’s typically a pre-bedtime activity.
In a pinch, grooming can also serve as a means of calming down a Daraz who’s having a freakout.
Forehead grooming is itself a dominance act (“I’m in charge here, and you need to settle down”), generally deployed by the older quad members to keep the younger ones chill.