This kind of tight monitoring is important as it closes an important loophole. In previous experiments on photons, there has always been the possibility that although the measured photons do violate the laws of classical physics, such non-classical behaviour would not have been observed if all photons involved in the experiment could have been measured. In the new experiment, this loophole is now closed. "Perhaps the greatest weakness of photons as a platform for quantum experiments is their vulnerability to loss – but we have just demonstrated that this weakness need not be prohibitive," explains Marissa Giustina, lead author of the paper.
I will not be surprised if we will soon see in one experiment where ALL the loopholes are closed simultaneously.