I think the contrived use case is just for illustrative purposes? If I'm understanding correctly, the combination of cleanup code and deallocation can sometimes consume enough time that it's worth dispatching it on another thread. That's hardly specific to Rust though.

As you note that will certainly add some overhead, although that could be minimized by not spawning a fresh thread each time. It could easily reduce latency for a thread the UI is waiting on in many cases.