Some studies have found that nonviolent protests are more likely to extract government concessions than violent protests.
Critics will often suggest that to be effective, protests must at least have the threat of violence behind them. Presumably, if politicians are scared that you might riot, then they will do whatever you want. What's never explained is why any politician would care about violence that doesn't affect them. Why would a politician care if I burn down a random person's car? Usually politicians just care about votes, and violent protestors have worse optics.
However, three potential justifications for violent action instantly come to mind:
(a) Burning down cars makes other people agree with your political positions.
(b) Politicians are scared of random people's cars being burned down.
(c) The violent protestors are planning to do something that actually threatens politicians, like overthrowing the US government.
I don't see much historical support for (a) and (b). Specifically with (a), burning down cars might in some sense raise awareness, but if most people disapprove of the violence, it would be an overall political loss. In the United States, the odds of overthrowing the government seem rather slim and not even like the aim of the protests, so that justification doesn't make much sense either.
So, despite all this, why do I say violent protests can be effective? It seems rather obvious that everyone has been going about this the wrong way. Protestors would be effective if they violently protested for causes they disagree with. Perhaps some already are. Why shoot yourself in the foot when you could shoot your opponent in the foot?