I can see how an instructor hitting a trainee in a class might establish a pecking order or serve as an initiation rite to a higher rank, but I fail to see how bruising someone's face or liver or thigh makes them a better fighter.
But I did see in the "Fight Science" documentary that Thai fighters can make their shin harder by repeated blows that cause the bone get thicker with calcium deposits. Likewise, added calcium deposits to your hands can help the striking surface to get thicker and harder.
Getting my face bruised in the 3rd grade by Randy Moulder just taught me to not to let it happen again! But it didn't help me better tolerate future blows to my face!
An infamous lady here at work likes to smack mens' butts
and bruise their ribs for play. If anyone would like to participate with "Large Marge" send your telephone number and she will be glad to help you acclimate to blows, bruises and other "submissions."