Who is it useful to and why is it better than the trades you can make with an ordinary brokerage account?

Keep in mind that much of finance is not obviously useful from an outside perspective, so "here's a trade you could do" is not enough to show usefulness.

I agree that this particular example may not seem useful (since there are other, centralized ways to trade AAPL derivatives), but the useful thing here is that it's all within the same ecosystem. I've traded for years using brokerages, clearing houses, and banks, and this removes all of that overhead. The pegs are maintained by arbitrage and this allows the removal of almost all of the overhead associated with futures markets. But that's really just the tip of the iceberg. I'm not here to convince anyone; you should convince yourselves that it's either useful or not. There's certainly a lot of real development going on if you know where to look, and the hype is warranted if you can see the whole picture.

If it were a system that handled all the taxes for you, that would be genuinely useful. Keeping track of it all yourself seems like it would be a drag?

24/7 open trade market that can't be closed or shut down, no middleman fees (just network fees which will trend lower as scaling solutions are brought online).

Stock trades are already decentralized. You can trade AAPL stock derivatives directly without using a blockhain or a stock exchange, so the blockchain part hasn't added anything useful to the transaction.

That's just simply not true. Stock markets / futures markets are not decentralized in any sense of the word. You still require a 3rd party if you're going to trade derivatives over the counter.

Tell us how you can trade stock derivatives without a intermediary like a broker.

It's called OTC trading. It's been a thing for more than a century.

Blockchain trading is simply a dark pool with less protections, and dark pools have also been a thing in finance for decades.