It's basically a psy-op. It's a way of making people think they've "done something" when actually nothing is ever challenged and it's excessively easy for those in power to completely ignore it.

I'm British and the iraq war protests (2003) being completely ignored despite looking like this went a long way towards radicalising me. I was 10 years old when I saw that on the news and I remember thinking for weeks how fucked it was that a protest of that scale can happen but the government still proceeds with whatever they wanted to do anyway. Definitely the moment I realised that you don't say "please" to power.