In this case it wouldn’t matter, but I wouldn’t be surprised if it was habit.

That doesn't happen in big unions, either. Two people with the same job title and experience would probably have the same salary, but the union will represent many different job titles.

It means, rather, that if Steve says "I deserve a raise", and the other union members agree, the union as a whole will say "Steve deserves a raise" to management. Management will negotiate with the union, rather than with Steve specifically. That means a professional negotiator, rather than Steve alone in a room with his boss.

And yeah, sitting in the background is the strike, i.e. everybody walks off if Steve doesn't get a raise. That's not as important as people imagine. It's a catchall way to understand what a union does, but the union has a lot of other tools in its toolbox. Some unions can't strike at all, but they shouldn't need to.