People join trade unions so that they are allowed to work. except
right to work states, which are in the south, unions have negotiated terms that require employers to hire from unions exclusively. This is the case with my union and most other unions in Washington. There are non-union shops in Washington mind you, but they pay their employees significantly less.
So again, from my experience with unions, people join them because they have to for proper employment, not for ideological reasons.