I would assume that the benefits are in having coworkers that fill in the gaps in your own business skills. For example: finding clients, invoicing, server maintenance, customer service, training, documentation, implementation consultants. There are a variety of job skills that one person will not have or want to contribute 100% of the time that are valuable jobs for junior or more senior coworkers.

If you are paying someone to do any of these jobs, you are doing it out of income that you've paid taxes on. If they are a part of your cooperative then it comes out of the businesses own funds pretax. There are also incentives to share in other resources, such as buildings, child care and other invisible labor that we normally place little value on.

Also, some cooperative companies will only outsource work to other cooperative groups.

Not to mention the camaraderie of working with people with similar goals in a noncompetitive environment where they value your success.