Brian Gaff (Sofa) was thinking very hard :
So what is this supposed to do? If its just so direction and hazard
indicators on the trailer work, normally these seem to be part of the
standard connection and run in parallel with the cars ones just like all the
other lights do. However if its an indication only that a trailer is hooked
up that would be easy to achieve with a link in the plug when its attached.

No, they are supposed to monitor the current, to provide feedback that
the trailer indicator is actually drawing some current/ working. No
buzzer/WL flashing = cable fault or lamp failed.