Why the ChatAgent drop the original response message content of model and replace it with FunctionCallingMessage with an empty content

[farmer00317558]

        assist_msg = FunctionCallingMessage(
            role_name=self.role_name,
            role_type=self.role_type,
            meta_dict=None,
            content="",
            func_name=func_name,
            args=args,
            tool_call_id=tool_call_id,
        )

This is a snippet from the _record_tool_calling method. Why is the content property not set to the actual output of the current step model?