To get a realistic number of fumbles as well as making every play a possible turnover. With it higher, I just wasn't seeing the randomness to fumbles on jarring hits. In other words, there is a reason teams kneel at the end of games. You will want to kneel as well if you can just run out the clock.
If you don't want to see as many fumbles, just raise the fumble slider a couple ticks, it doesn't take much for fumbles to all but disappear.
The AI RB runs fine with the lower fumble slider.