Having to adversarially game out every interaction just to thrive in the world, especially those interactions with entities that have a lot of power to make your life good or bad for a long time, is a serious burden to place on people, and unions can help to alleviate it.

I have also seen and read about the significant disparities in different demographic groups' (e.g. gender) willingness to do things like negotiate, and it can result in broad differences in outcomes.

(There's a lot of ways to work on that kind of issue, and I'm not necessarily trying to claim "unions" is the only or the best one, but they can help to mitigate problems of pay disparity.)