A SIP trunk gives you a termination point for SIP calls to a DID.
If you make calls suing standard SIP notification all the time, you don’t need a trunk, and you don’t even need a DID. But it can get tedious typing “firstname.lastname@example.org:5060″ all the time to place a call (and he never calls me back, either).
If you rent a DID (direct inward dial) phone number from a SIP service like Primus or Babytel, you can receive calls at that number, and use that number for CallerID. But you can only place calls under the providers calling plan.
Really, a SIP trunk is only practical if you have your own VoIP server, Asterisk, Talkswitch, whatever.
But once you do, you can use one number and make 20 simultaneous calls because YOU are the termination for that number, not Primus, etal.
In fact you could turn around and rent time on your PBX, with your trunk, and become your own mini-telco.
Hope that helps clarify things.