It's an example of the thunk pattern. Thunks are used when you have an async call, e.g. getFoos. Then your action creator would be

  function requestFoos() {
    return { type: REQ_FOOS }
  }

  function gotFoos(foos) {
    return { type: GOT_FOOS, foos }
  }

  export function getFoos() {
    return function(dispatch) { 
      dispatch(requestFoos());
      myFooProvider
        .getFoos()
        .then(function(foos) { dispatch(gotFoos(foos)); });
    }
  }

Really useful for writing reducers that want to keep track of when you're fetching data from the server.