They allow workers to stand up for their rights. Unions don’t take no shit from no one. In this situation, a company tried to file for bankruptcy, and not pay their workers their pensions, healthcare, back pay, etc. That’s illegal. They would have gotten away with it, too, except the workers occupied the factory and demanded what they were legally owed. The union provided the solidarity and organization for that to happen.
The Labor Beat video group is putting together a documentary about the victorious occupation of the Republic Windows and Doors factory in Chicago. The filmmakers were—unless I’m mistaken—the only media group given constant access to the inside of the factory during this action. They’ve put up a ten minute selection of footage on YouTube: