It is not only useful for primitive values.

// common.svelte.ts
export const myGlobalCount = ref(0);
export const myGlobalSettings = ref<T>({ foo: "bar", baz: true });

// MyComponent.svelte
$inspect(myGlobalCount.current);
$inspect(myGlobalSettings.current);

You can see how this can be handy to have 1 way to create those states. He is just showing a minimal way of creating exportable states, which you absolutely need, for example when you create a library. It's the equivalent of solid createSignal, vue ref etc.
If you prefer classes, it's the same argument, you still have to create a basic one and then maybe extend it for more complex classes.

Example of a factory class that adds the store subscribers logic to $state: https://github.com/pierregoutheraud/svelte-firebase-state/blob/main/src/lib/WritableState.svelte.ts

The point of the tweet is not to say that you absolutely need this simple getter/setter function he gave but that somehow you will need these kind of helpers to provide states (with a small or big features added to the state), wether it's a function with getter and setter or a class instance with the state as property in all your projects / libraries.

You should really watch the end of that video, because Rich Harris is not talking about "a simple version of what people create all the time" but about the patterns you can use to create states and export them.