From the docs of Gearman protocol:
SET_CLIENT_ID
This sets the worker ID in a job server so monitoring and reporting
commands can uniquely identify the various workers, and different
connections to job servers from the same worker.
So it does not have anything to do with worker-client relationship. That is only handled by the function handle that the client is passing and the worker is registering for. This ID is probably seen in administrative commands' output and can help you in debugging / monitoring your application. As a matter of fact, some interfaces (e.g. PHP) do not support this setting and are still fully usable.