Multiple possibilities. One reason for cops to do this is to give themselves an excuse to escalate the amount of violence they can use against protesters. Or as bait to rile up other protesters to follow their lead and then the cops who are close by or even the undercover regime officers quickly swipe the protesters away.

It is a tactic commonly used by the IDF against Palestinians: Israeli undercover soldiers seen arresting Palestinian protesters

Or they might simply be disguised to perform actions which would be illegal for the police while remaining masked and unidentifiable.

When the establishment does this, the infiltrators are called "agent provocateurs" and their job is to turn peaceful protests violent, or amplify the violence enough to justify the use of heavy/lethal force

Well let's just say that the Israeli elections are one week from today and "French Arabs" make a very convenient scapegoat.

Rubber ball or tear gas grenade for crowd dispersion