It’s the same people that don’t get socialised healthcare. Union fees are like a tax that’s used to pay people to do work for you, in the case of worker unions, people that negotiate working conditions, benefits, and wages, in addition to lawyers that can both offer legal advice and representation should you be in need of it.
Here in Sweden we have lots of different unions. We have a tenants union that negotiate rent increases, ensure that landlords do their end of the contract, and will help you with legal proceedings should it be required.
There are also various “customer owned” institutions, like my bank, insurance company, and the grocery store I get most of my groceries from. The goal isn’t necessarily massive profits, but enough to sustain and develop the services, with surplus going back to the owners, that is the customers.
It just makes sense. We all make use of schools as children, so funding that with taxes makes sense. It ensures everyone gets a good education, and at least one good meal per day. Same thing applies to healthcare.