If you excel, you are given the options for up negotiating your pay. You are given the option to negotiate your bonuses, supplementary pay, time off and what not.
If you're a "rockstar", congratulations! Your union has raised the bottom from which you can up negotiate your compensation. Your union will also advocate for things you, as a rockstar super employee, never will be able to. Like your benefits, working conditions and base pay.
If the engineers want to raise base pay, they're in an engineering union. They have no one to convince, but have the clout to make it happen.
If the engineers at firm X want a raise, they can negotiate with their employer for compensation with the added bonus of a higher base pay to negotiate from. They also have the added benefit of having the union advocate on their behalf if they choose.
You haven't worked with a good union. Good unions are known for their quality and skilled professionals, just like good companies are for their products.