"There's a sad miscomprehension in the larger political discourse that the primary purpose of labor unions is to demand higher wages and more expansive benefits. This is not true. The enduringly important role of unions -- wage increases or none -- is to give workers a voice in their company, and to imbue that voice with the power to force change. So 90% of what a union does is not bargain for better health care, it's file grievance claims on behalf of its workers. They demand better treatment, safer machinery, family-friendly scheduling, and equitable hiring. And they fight in the other direction as well, giving workers who may otherwise be ignored a channel through which to advocate for process improvements that would otherwise go unheard. Their day-to-day role is to give workers a voice in the workplace, and that remains even if they could never secure another wage increase again."
I'd link this to his point about political power:
"Absent a healthy union movement, the competition between the interest groups that actually govern our nation becomes merely a vying of different business interests, with few powerful forces advocating specifically for the interests of the working class. There is, of course, a free rider issue in the way unions work, wherein the entire working class -- of which only 8% are unionized -- can benefit from the health care expansions and worker safety regulations and guaranteed maternity leave benefits and all the other worker-friendly legislation the labor movement convinces the Democrats to pass, even as the average American doesn't realize it's unions doing the bulk of the organizing behind these measures."
Passing a law to secure a right doesn't actualize that right in people's lives. There are companies that routinely ignore labor laws, and because employees want to keep their jobs, they don't complain. Unions are the main mechanism by which workers secure their rights in the real world, as well as units for collective bargaining.