Over here the purpose of unions is to: Provide a strong enough legal response and guidance to deter companies from trying shady shit, pay better unemployment fees than the government and provide training/networking. They also negotiate collectively with the employers on behalf of everyone for things like paid sick leave, paid vacations etc.
I pay a flat fee every month because the union I'm in has always had relatively low unemployment, for others it's usually a percentage of their monthly gross salary (usually around 10-50€).
In what scenario would capping people's salary be good for the workers?