what exactly is the purpose of the iasign process in the "[ULTIMATE GUIDE] Jailbreak/Activate/Unlock Virgin iPhones v1.1.1"?

I am talking about the step that tells me to run "iasign --automatic iPhoneActivation_private.pem".
The reason I am asking is that even before running this, my iPhone already confirmed that it has been activated. I did not pay attention to the screen, but I guess it popped up the message right after I ran "killall shutdownd").
So what is the purpose of iasign?

Thank you...