The purpose of the sip id is to allow direct sip connections. It's possible to place a sip call with an identifier much like an email address. You could for example place a call to [email protected] rather than 61 2 555 12345.
It also allows extensions to be reached by dialling the SIP ID. If you wanted extension 2013 to be reached by dialing 0 (zero) then you could give the extension a SIP ID of 0. My understanding, and it may be incorrect, is that SIP ID is looked up before extension number or outbound rules. So...an extension with a "full" PSTN number as it's SIP ID will be rung rather than the call being processed by the outbound rules. It allows some interesting options especially when combined with call forwarding.