What is the use case for persistent derived stores?

[KevinAst]

I wonder if you could shed some light a question I have.

The common use case I see for persistence is writable stores ... so they will "pick up where you last left off" on an app restart, etc.

However I fail to see the need for a persistent derived store. Because a derived store is by definition a filter (of sorts) layered on other dependent store(s), it seems to me that you would want your persistence to come from the dependent store(s) rather than the derived store itself. Can you provide a use case for a persistent derived store?

I suppose I have a similar question on readable stores, although it may make more since to glean an initial readable persistent value.

Thanks for your time and your product.

[KevinAst]

In thinking about this more, I suppose you are merely covering all the bases where a store's "initial" value can be specified.

Certainly both readable/derived stores promote an API where an "initial" value can be specified ... although more obscure.

I am assuming that within the persistent store philosophy, wherever an "initial" value can be specified, it in essence becomes a "fallback initial" value, where any persistent storage entry takes precedence. Would this sum up the basic philosophy of persistent stores?

I guess my question is more for the native the svelte store API. It seems like "initial" values for readable/derived stores is an obscure requirement, because these types of stores are more "controlled" (than writable).

Can you think of readable/derived "initial value" use-cases that would make this more understandable?

Your help is greatly appreciated!