Under right to work, no employee can be required to pay fees to the union. Once provided with an out, many workers naturally choose to stop supporting it. Some may have never liked the union or its politics. But others may opt out simply due to economic self-interest: It makes little sense to pay the union for a service that it’s obligated to provide you anyway.
Conservatives like to say right-to-work legislation promotes individual freedom. Unions like to say it advances individual free-riding, since workers can enjoy the benefits of the union’s bargaining without helping to underwrite it.