All over the world, the tactic of mobilizing large numbers of people for nonviolent marches, strikes, picket lines & sit-ins has contributed greatly to bringing about important social & political changes.
Spontaneous demonstrations are generally unlikely to achieve their goals; an effective demonstration requires careful planning & organization, which often includes obtaining legal permits & alerting law enforcement officials to ensure the safety of participants. Violent demonstrations are almost always counterproductive.
Organizing demonstrations, training people to participate in them nonviolently, keeping spirits high among demonstrators, & maintaining a disciplined sense of collective purpose during demonstrations are all vitally important skills for a successful movement; & people planning demonstrations should make sure to draw on the experience of veteran organizers.
Some people doubt that demonstrations are an effective tactic, or are embarrassed to be seen participating in them; and indeed there are few political goals that can be achieved through demonstations alone. But undertaken alongside a broad array of other modes of nonviolent political action & protest, they can contribute a great deal.
So people who are concerned with helping to bring about significant change in our society & the world should probably approach the question of joining demonstrations in something like the way most of us approach the question of voting. Taken by itself, one vote or one more body in a protest march may be a small thing & a fleeting gesture. But it is important to do nevertheless, because it is a sign of commitment to & participation in the democratic process of bringing about change, because it encourages commitment & participation in others, because it lifts the spirits & recommits us all to the struggle, & because it is likely to have a small but cumulative effect on the change process as a whole.