By default, the agent does not allow transferring to agents that are offline.  Enabling this allows for you to use a specific skill to transfer the call to an agent's personal queue.  An example use would be if you have a custom personal queue script that allows the caller to leave a voicemail.  This field requires the Transfer Skill to be populated.  The skill number used should be configured to have the Custom Script property set to the CustomPersonalQueue script.