What do unions do?

Harvard Professor Richard B Freeman on Labour Unions 

The first thing unions do is to raise wages for working people, and that obviously benefits the working people. They also increase the kind of benefits that workers want. So, if workers want pensions, the unions negotiate for that. If workers want maternity leave, that’s what they bargain for. If workers want to have better insurance and are willing to give up some wages to get it, unions help them. Unions change the pattern of compensation towards greater benefits. 

Because unions make working life better for workers, they lower turnover in unionised workplaces. Employers with unions traditionally have workers who stay longer and contribute to raising the productivity of the enterprise. Employers also get more credible information about what workers really want in the workplace, because the union representatives are democratically elected and they really speak for the workers. So a good, functioning union is a real positive. Of course, not all unions function well. But our evidence, and the evidence people generated twenty years later, demonstrated that, on net, unions are a positive force in the economy.