There is a lot of truth in this statement by Justice Louis Brandeis. Without unions, the employer-worker relationship is a one-sided one -- without all the power being held by the employer. Many on the right think this is proper (especially those with Wall Street connections), because the employer is the one that has provided the investment money. But the truth is that the money cannot produce a profit by itself. The labor of workers is required to make that money earn money, because without that labor no goods can be made or services delivered. In a capitalist system, both money and labor are required to make the system work. And since labor is equally important, those who provide that labor should have some power also. And the only way workers can acquire any power approaching that of employers is to organize (form a union).
The sad fact is that Wall Street and corporate America have been attacking unions for years (through propaganda demonizing unions and by buying congressmen to pass anti-union laws) -- and far too many Americans have bought into that (and believe unions are either corrupt or unnecessary). Unless we can change this attitude of Americans and policy of government, wages will continue to be stagnant (which means that because of inflation they will buy less with each passing year) and the middle class will continue to shrink (since unions are the biggest reason this country once had a vibrant and growing middle class).