> Clearly they are making something of value in someone's eyes.
Most of what a successful hedge fund does is moving wealth rather than creating it. The people it's moved to will no doubt be very happy to pay for that service, but that doesn't mean any value is actually being created.
Imagine a world where money literally rains down from the sky. Anyone can pick it up and use it to buy things.
But some enterprising people think "Hmm, I bet I can get a larger share of this money". Some of them put up big poles and attach enormous nets to them, collecting money that would otherwise have fallen in other people's gardens. Some buy planes that fly around catching money before it has a chance to land. Some find ways to generate strong winds that blow the money to where they can collect it.
Much competition ensues. Higher, wider nets! Faster planes with broader-area money-collecting apparatus! Stronger winds!
After a while, most of the money falling from the sky ends up in the pockets of a few organizations that can afford (and have the skill to develop) the most effective money-collecting machinery (nets, planes, wind generators) and have managed to negotiate legal rights to collect money falling in areas they wouldn't otherwise have had access to.
They very kindly sell their money-collecting services (this is how they fund the development of better and better money-collecting machinery) and, indeed, they provide good value for money: you can often pay them $1M and get back $2M. They are, that is, providing a service that is valuable to their customers.
But they are not increasing the amount of money falling from the sky. All they're doing is catching it more effectively than anyone else is able to. They do very slightly increase the total amount of money being collected, because their machines can get at a few otherwise inaccessible places, but mostly all they do is redirect to themselves (and their customers) money that would otherwise have ended up in someone else's hands.
A successful hedge fund is a little like one of these money-collecting organizations. It has developed effective ways of making money flow into their pockets rather than others', it provides that service to its investors at a price they are happy to pay, it may create a very little value by increasing market efficiency a bit, but mostly it's playing a zero-sum game.
> Why does everything have to be objectively useful and beneficial to the whole of society?
Clearly it doesn't. After all, we have all these hedge funds, and the people running them and working for them are very nicely compensated for it.
> the simple question of 'is something willing to pay for it with their own money'
If you make that your only question then you end up approving of theft and assassination, both of which are things people are willing to pay for with their own money. Unless you're happy with that, you need some other principles.