Suppose I own 0.01% of the company I work at. If that company is publicly traded, I can sell those shares to anyone.

If that company is a co-op in which only employees are allowed to own shares, the only people I can sell my shares to are other employees.

In general, more willing buyers (who in turn know they can they sell those shares unrestricted in the future to any buyer), increases the people willing to bid on those shares at any given moment in time. (It’s the same basic reason that you’d rather have $100 in cash than $100 gift card for Starbucks.)