[Freeswitch-users] Newbie question

Hi, I am a newbie in this list and I am finding openser a lot along with freeswitch and asterisk. First what is the use of openser with free switch/asterisk, is it required with free switch if yes in what kind of environment. It helps in scaling the solution or if natting than its not clear. The purpose is not clear. Thanks & Regards, Sunil