They are necessary in speculative theories that aim to explain the cosmic excess of matter over antimatter, such as leptogenesis and baryogenesis.
Because of momentum conservation laws, the creation of a pair of fermions (matter particles) out of a single photon cannot occur.
However, matter creation is allowed by these laws when in the presence of another particle (another boson, or even a fermion) which can share the primary photon's momentum.
Currently, two-photon physics studies creation of various fermion pairs both theoretically and experimentally (using particle accelerators, air showers, radioactive isotopes, etc.).
It is possible to create all fundamental particles in the standard model, including quarks, leptons and bosons using photons of varying energies above some minimum threshold, whether directly (by pair production), or by decay of the intermediate particle (such as a W− boson decaying to form an electron and an electron-antineutrino).
According to the Big Bang theory, in the early universe, mass-less photons and massive fermions would inter-convert freely.