The link to the Wikipedia article explicitly mentions that is was originally designed to just coop up a crowd for several hours so that they get bored and the situation de-escalates. This was seen as a more effective and humane tactic than dispersing rioting crowds using fear by doing police charges. That said, 1990s era European protests (when the theory behind kettling was developed) were rather less violent than current day American protests, helped in no small part of course by the relative prevalence of firearms in American society and their higher distrust of police forces (again, compared to Europeans. Atm distrust between police and protesters in the USA seems to be in a vicious downward spiral where the actions of both group confirm the prejudices of the other group)