According to the labor theory of value:
When an individual negotiates and works for a company, they make the company X$/yr. And they only pay the person YX$/yr where Y is a value from 0-.99 . That's because the company relies on this imbalance across its workforce to extract money from employees' labor. Your value may be X$/yr, but you are compensated for .2X$/yr . The .8X/yr goes to the company as their profit.
In unions, the workers align themselves together in order to exert more fair (read YX$/yr where Y is closer to 1) wages. They do this because, as a group, they can effectively strike and threaten to cut the life-blood (extracted labor value) from the company for a time. In the end, the company still retains a significant amount of percentage from laborers and maintains the bulk of the money. And since there is still a monied interests vs workers condition going on, propaganda and laws can be affected by the owner class to disenfranchise the laborers. Example: look where the US is now, with anti-union, anti-worker, and "right to work" laws.
In a worker owned cooperative, the labor value is closer even than that of a union/company. There still exists a need to keep the organization going on (maintenance, upkeep, hiring), however the aforementioned strife is combined into the same group of people. And when profit is paid out, it is paid to the very people who worked for it. In other words, the labor theory of value approaches 1 under worker owned cooperatives. We have organizations like Mondragon to look at how they implemented this.