My union in Europe doesn't really do that much in my day-to-day work. As a skilled worker in demand we're generally treated very well.
But for the broader questions they are nice to have. For instance they recently spear-headed the abolishing of anti-competitive contracts. That's something a lone worker probably cannot achieve.
When I graduated they said what I should expect to be paid. Which was useful as I really didn't have that much information (compared to the companies), and it also helped to anchor the pay as no graduates would settle for lower. And the union could reveiew the contracts and see what we should push back on before signing. That in itself probably helps keeping everything straight, as the companies know they cannot get away with abusive contracts so they won't even bother trying.