I can't speak for NPC sisterhoods. But I believe most organizations have some form of Articles of Incorporation. Our small national organization makes mention of Articles of Incorporation - although i've never seen a copy of the document.

From what I understood in addition to the uniformity and establishment rights with Incorporation, I believe isn't it also to some degree for legal/tax reasons. Fraternities and Sororities function as non-profit organizations. And I think a national organization has to have some legal binding such as incorporation to function as such. Also i'm sure some other legalize exists.