I think they need to know what country you "belong" to as well as your passport number since the passport number doesn't tell them where you are from, and entry/exit regulations differ depending on your country! For example, my husband is American (USA that is!) and I am British. I therefore do not have my passport entry stamped when coming into an EU country, while he does. Conversely, when I leave the US (in two weeks time on the Navigator!! Hooray!!) that pesky little green bit of a form has to be sent to the US government - the cruise line needs to know this!!