According to wikipedia, photon counting is a technique in which individual photons are counted using a single-photon detector (SPD). However I do not know why we do photon counting because the terms used are out of my knowledge.
Basically, single photon detection allows us to detect very dim light with better resolution and less noise than a more conventional detector. This is good when the light is already really dim (telescopes) or we don’t want to use more light than we absolutely have to (X-rays). There are other applications too but they all boil down to SPDs basically being digital (“photon” or “no-photon”) compared to a more conventional analog detector.
Basically, single photon detection allows us to detect very dim light with better resolution and less noise than a more conventional detector. This is good when the light is already really dim (telescopes) or we don’t want to use more light than we absolutely have to (X-rays). There are other applications too but they all boil down to SPDs basically being digital (“photon” or “no-photon”) compared to a more conventional analog detector.
Edit:typos