Sometime, when a phenomenon is so well-known and well-understood, we often use it to study other things. X-ray diffraction is one example. Another is photoemission/photoelectric effect.
Photoemission is the extension of our understanding of the photon picture of light. Ever since Hertz's discovery of the photoelectric effect phenomena, Einstein's theoretical photon model, and Millikan's subsequent verification of the Einstein's photon model, this effect has been so well-tested and understood that today, we use it to study other things. In particular, photoemission, in its various forms, is used to study the electronic properties of solids, such as metals, semiconductors, superconductors, etc. In fact, the clearest verification of the validity of the band structure of solids came from photoemission spectroscopy.
The progress in this experimental technique evolved rather spectacularly after the discovery of the high-Tc superconductors. Having the 2D layers of copper-oxide planes where most of the superconducting effects are thought to occur, made them a natural candidate to be studied by photoemission, especially using a technique called angle-resolved photoemission.
It is imperative to point out that ALL of the theory of photoemission, including those applied in the study of materials that we are now using in modern electronics, make use of ONLY the photon picture of light. There have been NO other alternative formulation of light to account for the experimental observations of photoemission spectroscopies. NONE.
There are two very good reviews of the usage of the photoemission technique on superconductors. The identical technique is also used on other materials.