The unions are organization of workers against employer. They try to defend workers against abuse of employer that come from his advantage that come from what he has. As Marx said, wage labour is paid slavery. The the unions is the organization of slaves against it's owner.
That's all in theory. But we mustn't forget that when there are money there is almost nothing resistant for corruption. And the unions sometimes are corrupted too.
"Property is theft."
Pierre-Joseph Proudhon
"the system of wage labor is a system of slavery"
Karl Heinrich Marx