Why did companies want to crush labor unions?

Okay, so a labor union is basically a right to strike against a company,  or to stop working at a company and still get paid. If a company was treating a person unfairly, and he went to a union, the union may be able to just dismantle the companies unfair treatment, and the person is paid until they can find another job. I hope this helps! :)

Have a great rest of the day.