Getting arrested is often an important public relations component of a demonstration. The person being arrested (usually leaders of the demonstration) wants to "make a stand" for the publicity benefit. The police negotiate and cooperate in the choreography of the whole thing, usually as a means to defusing the situation and bringing the demonstration to a peaceful close.
Symbolic arrests don't always work as planned. We had a situation in Miami many years ago during a migrant workers strike where four symbolic arrests were negotiated (Caesar Chavez and three of his top people). A line was literally drawn on the ground, the TV cameras were positioned, and when all was ready Chavez and the others stepped across the line and were arrested. You've heard about the 10% that don't get the memo? About 90% of the demonstrators didn't get the memo! More than 200 people stepped across the line and were arrested and had to spend the night in jail because the organization didn't have enough money to bail them out. It was a mess, but everybody just laughed it off and worked harder on communications at future demonstrations.
I see, thank you for the explanation, I didn't understand what they were referring to with that.