Well, I assume, Caissa, that the purposes are the same as non-violent protest: a statement against some form of oppression. Like statica says, it is a 'diversification of tactics'; that is, a different way of manifesting the protest statement. I think it's interesting that it's mainly these anti-globalization, or anti-capitalist demons that most often result in violence, because they are direct responses to the violence enacted and enlisted by capitalism to promote its well-being. Anti-war protests, even anti-occupation protests (in the West--i.e. not by those being occupied) rarely escalate to violence.

That is not to say that the violence isn't occasionally or simultaneously satisfying to the actors in and of itself--as peurile retaliation or thrill-seekin--but contrast this with the fun many peaceful protestors enjoy at demonstrations: witty slogans, drum circles, artistic political expression, etc.