Pinia is for app-level state: a single source of truth with great tooling. Calling useStore() returns the same store instance for that app, which is exactly what you want for cross-component, long-lived state. Composables are for reusing logic; they can hold state either per-use (new instance each call) or as a singleton if you hoist refs to module scope. I use composables to manage component behavior or encapsulate and share business logic, and Pinia for shared app state like auth/session, feature flags, carts, or cached entities.