David Brin argues here that

They're are doing this not in order to escape government, but because we on Planet Earth appear to be heading, inexorably, toward a world government (WG).

Because of the way that WG is forming on Planet Earth... with the judiciary and bureaucracy first and the legislature last... the chief effect is to ensure that individual humans have no legal standing before international agencies. Only sovereign nations have standing, can file suit, negotiate treaties, assert rights and privileges. ... But here's the crux. If they can establish a dozen or so new, sea-based national entities, to stand alongside the 200 or so that already exist, then the SeaSteaders will be in the same position as the original founders of the New York or London Stock Exchanges.

They will have inheritable or negotiable "seats" -- a grandfathered position of "standing" allowing them to step up before WG bodies representing the interests of millions of clients. Large and small.