I mean, a union is simply a way for workers to organize independently from management. While it's useful for countering bad management and things like that, that's not its only purpose. And anyone saying that it is either doesn't know as much about unions as they think or is anti-union while trying not to say as much directly.

It also gives a voice to everyone and establishes a single bargain. If you have 50 employees and you’re negotiating with each of them separately how sure are you that you’re really being fair to all of them and not having any biases impacting your treatments? How sure are you that they agree? If you agree with the union about compensation and conditions of that compensation you know it’s fairer. If every management action has a steward as a witness you know that someone else who knows the rules you agreed to will speak up if you break them. You know that you aren’t about to get hit with an accusation of misconduct behind closed doors unless the steward agrees you misbehaved.

And as you say there’s security against management changing. You get someone with a union job with a pension and it’s hard for them to willingly leave. It’s a level of security that you know you aren’t getting elsewhere even if you’d be promoted.