A commonly cited example is during the Battle of Seattle the cops wanted to beat the shit out of a nonviolent sit in and the black bloc protected them through a combination of strength and diversion. The non violent people are there for the optics and the violent people are there assuring that any move made on the nonviolent protesters will be rewarded swiftly.

The important part is that the violence mostly doesn't start until someone tries to hurt those who are there peacefully. Good was there peacefully so retaliation is becoming a possibility.