The whole point
It is very easy to claim that demonstrations do not work. There is a sense in which they are all about putting numbers on the street and there are almost never enough, and even when there are enough, the police lie about it. But even the smallest demonstration is a signal that there is a point of view out there which someone feels the need to express and that is a good thing to register, even if the cause is a hopeless one.
It is easy to claim that people just go to meet up with friends and feel virtuous, to which I am afraid the response is Well! Duh! Of course you meet up with friends and even if you don't you end up talking to people and making new ones. And of course you feel virtuous - you are out there and telling the world that there is something to which you are refusing to consent and that may not be the most virtuous act in the world, but it is not exactly a wicked self-indulgence either.
And sometimes you learn stuff you didn't know, and sometimes you at least acquire a more specific sense of why Islamic fundamentalists or members of the SWP are pains in the ass, and sometimes you get pamphlets from strange and exotic sects like the Posadists.
The Posadists were Latin American Trots, who worked out that there must be a reason why the US government was suppressing information about UFOs, and that reason was of course that the aliens were here to spread revolution. After all, only a socialist society could possibly reach the levels of technology necessary to master inserstellar flight, right!
I dream that someday I will collect a handout from Social Credit people or from the sort of Earth Firsters who believe in reducing humanity to a few thousand hunter-gatherers the way we were supposed to be.
Human possibilities - that is what demonstrations are for, that and picking people up, which is a human possibility all of its own.