Private equity is good for society because it provides a financial incentive for owners of the equity to increase the value of a company. The value of a company is tied to how much value it provides society. Financial incentives do work in practice in affecting behavior in humans. Especially with the scale that some companies can reach. In conclusion the concept of privately held equity existing accelerates the betterment of society.
> The value of a company is tied to how much value it provides society.
https://en.wikipedia.org/wiki/Goodhart%27s_law