Because every citizen has freedom of speech, and in order to be heard people feel the need to go to public places protest about something, and make other people aware of the problem.
Once a group is protesting in a public square, for example, other people that didn`t know about the problem can now know it, and also protest.
Also, it is a way to be efficiently heard by the government, because usually the media records the protests, broadcasts it on television, and rapidly the problem draws a lot of attention.
