I was a member of a Union for several years. Most illuminating. The main point that I took from the experiences was that if you don't get involved, you deserve whatever happens to you. And the main purpose of the union is to work for the interests of the members, and to protect them from different levels of management interpreting the rules in different ways.
One of the better things unions do now is act as the filter between pilots and management on the Flight Operations Quality Assurance program by being the ones who review the FDR files. Works well in the big airlines and those helicopter operations that have 'em.