That really depends on your industry and your union (and where you're based). My union doesn't negotiate my salary, I do. They do provide help with contracts, NDAs, legal advise and a bunch of other stuff and do provide salary guidance. They are also cheap at ~€475 a year.

Another larger union, which organises industrial workers, cleaning staff and generally people with less formal education, is almost twice the cost. They do negotiate at least base pay for the industries they represent. Many of the people they represent are often better off having their union do the negotiations. When handling negotiations it's obviously not only about money, but the unions do need to be able to provide at least raise in salary that can cover their dues, and sometimes they can't.