Unions are also formed to prevent large corporations from colluding to keep labor costs artificially low. (Yes, large corporations do that. Recent example: leading technology companies in Silicon Valley who colluded for years not to poach employees from each other, keeping compensation lower than it would have been if there had been true competition for employees.[1])

[1] https://en.wikipedia.org/wiki/High-Tech_Employee_Antitrust_L...