Unions (usually) put a floor on your salary, not a ceiling. I'm a member of a union, and my salary is a good 20-30% above the union minimum.

I've never had to contact my union (though I've benefited from it indirectly), but my sense is that they're useful as advocates for their members interests, especially in larger organizations. Often, this can even be for simple "common sense" proposals that are not controversial, but are difficult for individual employees to make happen. Navigating the bureaucracy of large organizations is not easy, and unions help with this (by creating an additional bureaucracy admittedly, so they're not a perfect solution).