Protests provide a radicalization opportunity, an opportunity to seize narratives, and an opportunity for organizations to connect and gain new members. They're pretty much useless for affecting the government, as others have pointed out, but that's kind of the point. When hundreds of thousands of people are marching in the street for months and the government is still saying "Yeah, we don't care. We're just gonna keep bombing brown kids instead of fixing homelessness" then that's kind of hard to ignore for people who are just coming into their political consciousness.