Foam density is graded from the crown down to the cheek panels, since a uniform-density guard either feels bulky at the temples or under-protects at the chin — getting that gradient right is what separates a comfortable sparring guard from one fighters avoid wearing.
Boxing and MMA gyms running regular sparring sessions are the primary buyers, and the open-face vs full-face choice usually comes down to whether the academy trains striking-only or includes clinch and ground sparring.
Let the interior padding air out fully between sessions rather than storing it inside a closed bag — trapped sweat is the main cause of the foam breaking down faster than expected.