TLDR, for people like you and me, who want to do useful things that help people, money can be very useful for that.
For an extreme version, look up Effective Altrusim. Some argue that the best way you can help others in this world, is to earn as much as possible (for example at a trading desk) and then give all your surplus earnings to the most charitable causes.
Personally, I would like to earn money so that I can work on projects that I think are worth doing and have a chance of using my particular skills and drives to good effect.
For example, I feel ready to take open hardware to a new level by building a small-scale chip (IC) factory capable of manufacturing open source designs (or any designs) at a realistic cost for customisation. And that's just one step in a series of related things, each of which will contribute something to the world if they are successful. That's just an example: Other people have their own grand projects they think are worthwhile.
But that venture is both challenging and expensive, and I'm still paying off debts from the last failed bootstrap. So my next big project is paused, until I have earned enough to be able to bootstrap all over again.
Something like HFT would be rather useful at providing the money, and its on my list of things to explore in the short term, because it's also a good match for my experience in several areas. But it's not something I'd want to do for a long time for its own sake. It would be for the money, so that I can do other useful things down the line.