It’s not always necessary to ‘go through the union’ instead of speaking with your manager. For pay, conditions and other disputes, yes, you’d want the collective bargaining of a union to get the best deal. For other things, it’s not always necessary.
I had a fall at work a couple of years ago. I fractured my ankle and was away from work until it healed and I could work again. When I returned, the problem area that lead to my fall hadn’t been looked at. I raised it with management myself and they dealt with my concerns and rectified the issue. I informed my health and safety union rep of what I was doing and he agreed to take it up if necessary. It wasn’t.
The company I work for has a long history of working with unions. They obviously have their motivations to improve efficiency and profitability but generally aren’t arseholes about it. The toxic culture I see in other companies only comes about when bosses can get away with murder because employees aren’t able to stand together.