Protests serve the same purpose as newspapers, blogs, Facebook pages, Twitter feeds, phone calls, letters, etc: they're for communication.

You might not be able to afford that billboard on the street corner, but you can afford to stand there with signs and still attract attention. Protesting is mass communication for time instead of communication for money.

Personal example: Nearly every time I've seen a group of picketing workers outside an office building has been the first I've heard anything of their employers' practices. Even if I might care as a customer, why would I know about it?