[Nullabillity]

You can implement futures by just running each future as a thread, but it doesn't really give you much. It's a lot more complex to write a preemptive thread scheduler + delegating future scheduler than to just write a future scheduler in the first place.

Especially when that future scheduler already exists and works, and the preemptive one is a multi-year research project away.

[pron]

It gives you a lot (aside from the ability to use existing libraries and APIs): observability and debuggabillity.

Supporting tooling has been one of the most important aspects of this project, because even those who were willing to write asynchronous code, and even the few who actually enjoyed it, constantly complained — and rightly so — that they cannot easily observe, debug and profile such programs. When it comes to "serious" applications, observability is one of the most important aspects and requirements of a system.

Instead of introducing new kind of sequenatial code unit through all layers of tooling — which would have been a huge project anyway, we abstracted the existing thread concept.