I mean in theory nothing. But then that dialogue would have to be called something, and it would basically be the union at that point.

I mean that's all the union is, a group of employees having a unified relationship with the company.

I was working in a factory at the time, and the nature of that work didn't lend itself to individual bargaining, like say software engineering. That said I don't see why you couldn't ask for more if you're a better employee, I never got told I couldn't ask. It might be down a different path, like a promotion. And not every union would necessarily do any salary negotiation.