They are for legal residents without the ability to get proper documents from their home countries. My guess is primarily "refugees" that haven't gone through the full citizenship process.
Got it, thanks, in hindsight, I should have googled it myself. I've never seen a refugee travel document / refugee passport be called an "immigrant passport" before, but that's cool, and more importantly, it's great to see Norway hasn't created a "second class citizen" category, and that all citizens are truly equal, at least from a Passport/Documentation perspective.