There is another use case for async in client-side JavaScript that often gets overlooked: modeling the flow of UI.

So, let's say you need to show a "dialog" which, upon exit, should execute a piece of code (e.g. to show another piece of UI). Maybe that dialog can be invoked from multiple places, and the code to execute (after the dialog) changes accordingly.

You could do that via callbacks, but that is not very composable, and is generally messy.

Better solution: a Promise which gets resolved when the user exits the dialog! Composing multiple pieces of UI then becomes equivalent to composing multiple (async) function calls, you can pass output from one as the input to another, use exceptions to "circuit break" the UI flow, etc.