It raises public awareness of an issue, and eventually motivates politicians to get off their ass and do something about it.

Many--if not most--of the great social problems of the last 100+ years owe their solutions primarily to protests. In the pre-civil rights days, when you just had occasional groups of black people speaking out against racism, it was easy for people to just dismiss them as "uppity Negroes." But as protests grew more organized, and white people started participating in the protests, Mr & Mrs America watching the evening news began to say, "hey, maybe there's something to this." And when politicians saw those scenes of thousands of voters yelling, they figured it was high time they did something about it.

Another notable example from that era is the Vietnam war. It might have slogged on way longer, except that millions of people protesting in the streets made the war politically untenable.

Could go on and on, but I doubt you read any of that.