It's private because there's no reason to make it public: it would be odd for another object to log a message via the worker.

Or did you mean "why's the reason for making this method at all?"  (I wasn't sure what you meant...)

It adds the pid to every log message.  When I was troubleshooting stuff, I found it very useful to have the pid included in the message, so that it was clear which process the messages were coming from.  I thought it was useful enough to keep.