In a union, the leadership is negotiating for you. So the person in charge of your employment that the union is replacing is you, not management. They could negotiate things like more PTO, better conditions, or a larger bonus instead of a straight base pay. But as the employee you might want straight up higher base pay. Or maybe the union gets a promotion process that values seniority more than performance. It might save you from a political process or having to play the game, but can hurt other people.

The idea is that unions help the collective and will get a generally better deal then you might get on your own. But again the isn't going to get everything from management and they have to decide what to prioritize. So if your priorities are different than union leadership your wishes might get left behind to help more other people.