Why Asterisk destroy users and registry on sip reload

every time i issue the command sip reload and if their is some change (even the timestamp by using touch command) in sip.conf then asterisk destroy all the registry and re-register with new call-id.
why is this necessary to destroy all registry and re-register again.
According to rfc3261 call-id must be the same for all requests
and responses sent by either UA in a dialog. It should be the same
in each registration from a UA.