Can be used to detect single photons.
Richard Feynman likes them, he describes the tube at Richard Feynman Quantum Electrodynamics Lecture at University of Auckland (1979) at one point.
It uses the photoelectric effect multiple times to produce a chain reaction. In particular, as mentioned at youtu.be/5V8VCFkAd0A?t=74 from Video 1. "Using a Photomultiplier to Detect single photons by Huygens Optics" this means that the device has a lowest sensitive light frequency, beyond which photons don't have enough energy to eject any electrons.
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A photomultiplier tube (PMT) is a highly sensitive electronic device that detects and amplifies light. It is commonly used in various applications, including particle physics, medical imaging, nuclear and radiation detection, and fluorescence spectroscopy. The PMT operates by converting incoming photons into an electrical signal, which can then be measured.