It frequently happens in classical relativity that it's much easier to carry out certain reasoning by talking about photons.
An example of some historical interest is in Einstein's 1905 paper on SR, section 8, "Transformation of the Energy of Light Rays. Theory of the Pressure of Radiation Exerted on Perfect Reflectors," where he says, "It is remarkable that the energy and the frequency of a light complex vary with the state of motion of the observer in accordance with the same law." If you already know $E=h\nu$, then you know trivially that $E$ and $\nu$ have to transform the same way. Since Einstein was establishing it for the first time, he had to go through a lengthy argument about the transformation of the electric and magnetic fields. I think this was around the same time he was working on the photoelectric effect, so he must have realized that this was necessary if his theory of light quanta was to be consistent with relativity.
A similar example is the derivation of gravitational time dilation using the standard thought experiment about a photon being emitted and received at the floor and ceiling of an elevator. I find it a lot easier to talk about this example by talking about a photon, although you can certainly get the same result without quantum mechanics.