I was told by my Practice Nurse when I got mine that it's in case I am ever hospitalised or in an accident, so the health care providers have access to my insulin details in case I'm incapable of telling them.
So it's got my insulin/diabetes details on, my GP and my next of kin on, as well as my usual hypo treatment. I keep it in my purse, where my ID would probably be kept so it would be found fairly quickly.
I find it reassuring as I have nothing "official" stating I am a diabetic, and people may not recognise or see my BG monitor or insulin pouch. But if I was in an accident they'd probably look for ID pretty quickly so it'd be seen.
I get my local pharmacy to collect my repeat prescriptions from my GP and then collect my insulin etc when it's ready, and I've never been asked to show my Insulin passport when requesting or collecting a repeat prescription.