Although unions have their flaws, they do help to keep wages high for skilled workers, both in terms of the prevailing wage but also in enforcing overtime rules (double time for Sunday work, for instance). They also prevent out-of-area workers from coming into an area when times are good. Skilled union workers in the Bay Area can work 168 hours per week right now if they want to.
The primary mechanism unions use to keep wages high for union members is to limit supply by lobbying to keep "un-certified" or "non-union" members out of the market. This provides a nice buffer, keeping the poorer class poor and protecting the middle class.
Nobel Economist Milton Friedman, for one. https://www.youtube.com/watch?v=xzYgiOC9cj4 If you watch to the end he fully explains how the net effect of unions is to raise the pay of high paid workers and lower the pay of low paid workers.
You might also like "Basic Economics" by Thomas Sowell which talks about this phenomenon also.
