I don't quite get it either. I usually do research before hand, and if it's significantly cheaper, I book my airfare independently, which is what I have done my past 2 cruises. I know Royal Caribbean offers custom air arrangements to those that are eligible (Diamond Members, for example), but I don't know about Carnival.
2007 - Majesty of the Seas (Bahamas)
2009 - Enchantment of the Seas (Western Caribbean)
2010 - Freedom of the Seas (Western Caribbean)
2013 - Golden Princess (Alaska, roundtrip from Seattle)