The discovery of the photon was one of the major initiators of quantum mechanics.
Light was very well known to be a wave through diffraction experiments. So how could it also be a particle???
This was a key development for people to eventually notice that the electron is also a wave.
This process "started" in 1900 with Planck's law which was based on discrete energy packets being exchanged as exposed at on the Theory of the Energy Distribution Law of the Normal Spectrum by Max Planck (1900).
Inward Bound by Abraham Pais (1988) summarizes his views of it:
Planck's on the discovery of the quantum theory (1900); Einstein's on the light-quantum (1905); Bohr's on the hydrogen atom (1913); Bose's on what came to be called quantum statistics (1924); Heisenberg's on what came to be known as matrix mechanics (1925); and Schroedinger's on wave mechanics (1926).