It's a way to keep other players honest.

In a game where your career, or your life, could be over in a matter of seconds over a single hit, the idea that you could get your teeth knocked out by someone for it is at least a slight deterrent.

When the fines amount to little more than pennies and suspensions rarely go beyond a handful of games, having to answer the bell for your actions can help keep a player honest.

And people may rarely get injured fighting, but try and tell me someone 6' weighing 200lbs punching you full in the face isn't going to hurt.

Same reason nerf bats are allowed.

Theres a documentary that will at least shed some light on the psychology of it for you called "Ice Guardians" its genuinely interesting.

But to put it very simply the refs cant see everything and its easy to make an attack thats outside of the rules look like an accident. If "A" team has a little guy who is exceptionally skilled the "B" team has literally no reason to not play dirty and attempt to injure him out of the game. Especially with the speeds the players are moving a "get him out of the game" hit could become a permanent life changing injury. So you put your enforcer on the ice and he will have a word "If you guys try and take him out again, I will punch the fuck out of the guy who does it in front of all these people, You have been warned" pretty simply, Hockey players police themselves and its unlikely that an on ice fight will lead to long term injury but the fans will see that beatdown and they know why. Keep it clean, keep it about the hockey or we can make it about the other thing.

Its a lot less prevalent than it used to be, which is probably a good thing. Its also a lot less necessary with all the modern high def cameras on a NHL game. But sometimes someone needs a message delivered in a very tactile fashion immediately not 6 weeks after the game after a tribunal has reviewed the footage and your lead goal scorer is still recovering from a severe concussion and may never play again.