Mainly this avoids the hell that global state SPA patterns produce: redux, reducer patterns in general, and 8 thousand context providers.
I do think there's use cases that warrant global in-memory state, but it's such a pain in the ass to maintain and evolve, i'd always plan against it. Every html node in your app does not need to know about literally everything going on and react instantly to it. it just doesn't.
Just make another page!
Also: so the islands pattern can be as fancy or rudimentary as desired. they can bootstrap themselves via async endpoints, they can be shipped as web components even, or they can be static, pre-hydrated in some manner.