. Socialization: some ppl jus join in protest to meet new ppl, get into new relationship and all that and if possible they might meet their soulmates..
. Means of Distraction: some ppl use protest as a means of distraction to steal. Cos everyone will b focused on the reason behind the protest and won't be fully conscious about the valuables they carried along with them.
. Also some terrorist may plan on destroying infrastructure using the protest as a cover-up.