Data Constructor Services are meant to be used as place holders for actual integration services supplying the data eventually, they initially populate hard coded data to the Business Objects for playback and demo purposes and quick prototyping, without any changes to upper level services the actual data integrations can happen at later stages.
Also it is not a recommended practice to have default values for your business objects, thus for unit testing you can use the plugged in data constructor service to provide the test data also.