Unions give workers a grievance voice to management that does not otherwise exist.

Contrary to MYTH. Wages are not the primary focus of unions. (especially sence Reagan).
They exist to correct excessive behavior and the ultimate power of management, and to regulate the expectations from employees.
Wages and perks are mostly set by the historic customs of the specific industry. NOT by unionization.
Many Business's/Orginizations have there own 'baggage of history' to carry. Teachers, Doctors, Lawyers to name but a few.

I wasn't  a Union manager, but my mother was!
 
:)