Many tankless units are not equipped with a buffer tank whatsoever so the small one within the 240A is a major enhancement and benefit.
Since a tankless unit's burner will not fire until water is already exiting the WH and flowing out to a fixture, even the small internal buffer tank will buffer the initial slug of cold water that will exit the heat exchanger before the burner achieves full flame. If you are planning to recirculate hot water just prior to when it will be needed (demand) or continually, then the recirculation loop filled with already hot water will function to enhance the capacity of the buffer tank.