It's never just about the protest. Protests are usually part of a larger effort. The efficacy of protests varies. They can yield immediate results, they can take years to build momentum, they can be quashed or quickly forgotten. but at the very least they let a community know that some of it's members care about something. It connects people and fosters further work on the issue. If anyone else in the community is concerned about this thing but isolated, now they know they have a community and path to doing work on the issue. It's a single part of many types of effort and activism that can bring eventual change.