The transformers are probably part of the self regulation system. Without seeing the circuit diagram I'm supposing the "no load" field excitation current is from the AC circuit via the capacitors and diodes to give DC . Under load, once output current starts flowing through the transformers it induces a secondary current in the excitation circuit which is added to the "no load" current flowing through the capacitors, is rectified and increases the field strength to maintain the output voltage. The greater the output load, the greater the excitation current hence the self regulation.