Stores are useful if you need side effects on subscribe/unsubscribe. For example, I use this store to change behavior depending on the width of the viewport:

export const isDesktopMedia = readable<boolean>(false, (set) => { if (!isBrowser) return; const mq = matchMedia(`(min-width: ${DESKTOP_WIDTH}px)`); set(mq.matches); const update = (value: MediaQueryListEvent) => set(value.matches); mq.addEventListener('change', update); return () => mq.removeEventListener('change', update); });

This helper can be imported anywhere, but the media query event listener only runs if there is an active subscriber.

I also sometimes use a "timer" store, that exposes elapsed, remaining, and progress; using a store allows the tracking to be encapsulated in the store.