If you are seeing a psychologist and gp who does medication, then how does a referral to a psychiatrist change things, why would this be necessary ?

More expertise: Psychiatrists are specialists with years of specialty training including psychopharmacology that a GP does not have. A GP is trained to refer to a specialist for many reasons if the patient requires more expertise, eg. surgery, GI, neurology etc. We specialists often work closely with a GP to give patients the best possible care. Hope this helps.