Comment by vhost-

Because then your caller will need to implement something that understands what a promise is instead of just getting a data object it already has to understand. Not only this, but the caller will need to also implement a polling mechanism to keep trying. Put into code:

  func get_value(key):
    value = backend.get(key)
    return value

Is way better than something along these lines:

  func get_value(key):
    while true:
      response = backend.get(key)
      if response.type == "promise":
        sleep duration
        continue
      return response.value
 

If your service looks like the former, then suddenly your unit tests can use a postgres database, a sqlite database, a rest client... that all implement the same backend.get(key) interface.