They're there to sheepdog the voters into believing progressive candidates a valued part of the democratic party despite the fact that more than most of the good things progressives have accomplished are entirely outside of the chambers of congress. The democrats refuse to work with progressives unless they are forced too, like that time the progressives asked Pelosi to consider stock trading bans and she denied them until it was clear that was horrible PR, then decided to come around to vocally supporting it later, once the political will to pass the bill was dying down. Progressives are there for show.