What is the purpose of transformers on the output side of a DC power suppply?
Transformers don't work on DC, so they're most likely common mode chokes.

A common mode choke is a transformer, but turned 90° so the current flows through both coils, in opposite directions, so the magnetic flux cancels, which does not saturate the core so it can be small and cheap, and presents a low impedance to differential current.
However, to common mode current, which flows in both wires in the same direction, the transformer coils are in the correct direction to create a high impedance.