I once had a legendary weaponsmith; he had been struck by a fey mood, rounded up a few bars of steel and some diamonds and proceeded to make a beautiful mace. He was married to a nice young dyer, had two kids; productive, well loved. Dozens of friends. Enjoyed helping others. When he wasn't producing helpful weapons for my military, he was always giving food to the wounded; rarely partied. Upstanding citizen material and he had a whole great life ahead of him; probably in the running for mayorship.
A woodcutter, a loner, a drunk who hated others got angry from being out in the hot sunlight and a lack of work. Friendless, one day he just lashed out. Carrying with him his steel battleaxe (a beautiful weapon, lovingly prepared by my weaponsmith to be used for good) he smashed down a door in an area near the forges.
I did not have a fortress guard. I had been told I could increaes the happiness of a fort by not having one. Oh I was wrong.
The woodcutter hacked down the door; in a policed fort this act would have brought a horde of guardsdwarves down on him, but in this one nothing happened. Did he feel any better? It didn't matter.
My weaponsmith rounded the corner, carrying a steel bar in his hand. He just had one more pick to make (a mandate for the mayor, but my smith didn't mind, he liked the mayor and picks were a sensible thing to ask for) and was looking forward to finishing his job and going home to his family. In the next hallway he noticed their was a missing door and a woodcutter; odd, as the woodcutters rarely came into the forge areas.
The crazy woodcutter charged the unsuspecting weaponsmith, slicing off his left hand. The smith, tough, didn't fade out quite yet and but tumbled to the floor. At this point, guards should have been everywhere, carrying the woodcutter to prison and helping the poor weaponsmith to a bed. No one came.
The woodcutter hacked off the smith's other hand; he would never forge a masterpiece short sword again; never haul a steel bar; never carry a wounded friend to safety again.
The woodcutter, satisfied that he had 'started a fist fight' wandered off, happy with the massive destruction he had caused. While he, completely unpunished, drinks sunshine, parties and takes breaks, my poor smith will rest for the rest of his life. The woodcutter will not spend a single day of his life in jail, or even feel the lightest beating.
This, my friends, is why we have fortress guards.
(in case you were wondering none of this happened but it could have; which is why I have never had a fort without a huge guard force)