One typical benefit of unions, beyond compensation concerns, are better-defined circumstances under which you're able to say "no" without repercussions, and the security of others having your back if management tries to wiggle out of their side of that agreement.

[EDIT] in short, in some situations you get to replace the typical "well, just leave!" response to management trying to fuck you over, with "... don't leave, because they won't be able to fuck you over, and if they try they'll be the ones having a bad day". Which is nice.