The point of unions is that individual workers can be easily screwed over with no repercussions for the company because of the power differential between companies and their workers. Unions seek to level the field a little bit by giving the workers collective bargaining power which allows them to secure better pay/benefits/influence the direction the company is going. When they work it is a very rational arrangement for the workers, so companies tend to not like them as it decreases their power over their workers.
Precisely, you can end up having people working for a company under terms that are technically legal, but still exploiting the weakest. The strongest people easily move on to better positions, but some, single parents, less skilled, psychologically weaker individuals and so on are more easily pressed into working conditions they may not like, because they fear losing the job they do have.
In the US the minimum wage haven't moved in decades, at least not significantly. Denmark doesn't even have a minimum wage, yet people are better paid, that's due the Danish unions. Interestingly the Danish unions are actually oppose a minimum wage, because they believe it makes it easier to legally pay people less.
