It’s a potential long-term persistent identifier. Safari already deletes first-party cookies from sites you don’t regularly access so they need to review any other storage mechanism to ensure it doesn’t turn into a replacement with different policies. That’s also why they have a different policy for a PWA, where the user has expressed a stronger intention to use that site regularly.
In this case, they implemented it following the same storage standard for everything to avoid inconsistencies.
> This feature can actually benefit privacy, because it allows creating web apps that don't have to send data back to a server to store it.
It’s more convenient in some cases than the other APIs for doing that, but it’s held to the same privacy policies. If you want persistent storage, you have to ask the user for it.
In this case, they implemented it following the same storage standard for everything to avoid inconsistencies.
https://storage.spec.whatwg.org/
> This feature can actually benefit privacy, because it allows creating web apps that don't have to send data back to a server to store it.
It’s more convenient in some cases than the other APIs for doing that, but it’s held to the same privacy policies. If you want persistent storage, you have to ask the user for it.