Doing something positive-sum is a way to become a billionaire, but many people are very handsomely paid to ensure that their clients are on the good side of zero-sum transactions.
Where "well-managed" means "good at delivering money to its shareholders", which is at best obliquely related to a positive impact on the world. (Also, I'm skeptical that the stock price in itself makes that much of a difference to what the company is able to do.)
> Another net positive of stock trading is making buyers and sellers available all day, for anything on the market. Yet another benefit of stock trading is to make it more difficult to manipulate the market - there's a reason why pump and dump scams only occur with assets that see very little attention (i.e. are low-volume).
In other words, the benefit-to-the-world of stock trading is that it makes it easier to trade stocks? And both of these are the result of unprofitable trading as much as profitable trading, so they can't be the proof that the money comes from the value delivered!
But stock trading also penalizes well-managed companies in slow-growing or more mature industries by giving them a higher cost-of-capital, just as it would if they were poorly managed. It seems a haphazardly blunt instrument for allocating liquidity to value generation. It piles on where rewards seem ready to be reaped, and makes it harder for mature sectors to renew or reinvent themselves.
That money management service includes, amongst other things, picking stocks. But that's the tactical "what". The value-added element is that he preserved and grew that wealth instead. It turns out that is hard to do, and is indeed a positive sum service to customers. They get to relax _and_ make money. Great outcome.
Only if the shares are newly-issued, though.
Usually your counterparty is just someone with different cash flow needs, or who disagrees with you about the future. No benefit accrues to the company.