Of course they can. There's quite a lot of apathy in this country and people assume nothing will change, but it can, but at the very least it brings about awareness to a cause and may spark interest in some people, even if a handful.
Some of them can feel like you're shouting in the void, though: e.g. anti-Trump, anti-war, anti-austerity. But it's important to get your feelings out and at worst it's usually an enjoyable day with good people, sometimes some drinks and maybe even some music.
Boots on the ground is a lot more important to me than voting for whatever political party as the system regardless is rigged.