My first exposure to BJJ was an extremely humbling experience. I was clearly stronger than anyone in the room and figured I could at least hold my own with this grappling stuff. Boy was I wrong. I could do nothing but accept the dozen different ways they tied me into a knot. . .and these weren't black belts. It doesn't take very much training for a reasonably fit person to be able to destroy someone who hasn't trained. There are a lot of people out there who have trained for a few months and dropped out for whatever reason and many of them are criminals. Training is important not only for you to know what to do, but also to recognize what they other person is trying to do to you. If you can tell in advance that the other guy is trying to go for a chokehold, or to break your arm, might that change the level of force you're allowed to use?