I think gtk-rs is another interesting example. The glib crate provides an async executor, since glib already requires running its main loop. So if you just need to run a couple futures you don’t need another executor in one or more other threads, and tasks spawned in the glib executor can call functions that need to be run in that thread, since gtk types are generally not thread safe.

So integrating into other things could be a reason to use a different executor…