OP's technique is useful for a consumer of a another function, to consume it in a synchronity-agnostic manner. In the implementation of the function consumer, you should find the usage of OP's technique.

In other words, this technique allows a library's implementation and interface to be synchronity-agnostic, but it does not say anything about the library user. If the library user likewise makes use of the OP technique, the library user code will remain synchronity-agnostic, otherwise it will be tied to be either synchronous or be asynchronous (parametrised to the synchronity).