What does 'Enable global server wide lock for CiviMail' do?
From the comments in CRM/Core/Lock.php:

   * @param bool $serverWideLock                                                                                                    
   *   Should this lock be applicable across your entire mysql server.                                                              
   *   this is useful if you have multiple sites running on the same                                   
   *   mysql server and you want to limit the number of parallel cron                                  
   *   jobs - CRM-91XX                                                                                 


So basically if you are running multiple civicrm instances on one box and want to limit the number of mailing jobs happening across all instances. This was funded by NYSS and was applicable to their installation.