I would like to know what all can be done using  openser/kamailio as a registrar server alone. i.e. in the real world scenario how does openser can be useful as a registrar sever alone.I would  like to make myself clear by pointing to the below example. i can integrate openser and asterisk and make openser as a registrar (only)server and can make use of asterisk for call functionality purposes.. Similarly, i would like to know where openser fits in (openser should only be used as a registrar server again).
Your guidance is greatly appreciated.