This circuit monitors the waveform of the connected mains supply. It does so in an isolated way so that the waveform can be recorded by an isolated, low-voltage data acquisition system. This is used as a "veto" signal in the lab to determine whether fluctuations in our sensitive instruments were caused by something intended or rather by a power spike on the mains supply to the lab.
After I built this, I discovered how rubbish the waveform from board mount transformers can be. If you can't trust the transformer, you can't trust what the circuit says the mains waveform looks like. Instead, I put the circuit in a bigger box with a toroidal transformer, with wires connecting the transformer to the board in place of the sockets for the board mount transformer. This worked much better.