Any time you have a barrier between one function being able to call another. The original article on this called them red functions and green functions. A green function can call a red function but a red function can't call a green function.

In terms of async, it's when you have to have a function with "async" attached to it and making it so that only other async functions can call async functions.

It ends up creating a weird circumstance where you can end up with a lot of duplicated APIs, particularly in libraries, because you are providing both async and non-async versions of functions.