Why does Strict Mode call component functions and certain effects twice during development?

This intentional double calling helps reveal components or effects that are not properly written to handle being interrupted or repeated, which matters for React's newer concurrent rendering features. If running something twice causes a visible problem, it usually means that code has a hidden bug, such as an effect missing its cleanup function.

useEffect(() => {
  const subscription = subscribeToUpdates();
  return () => subscription.unsubscribe(); // without this cleanup, Strict Mode reveals a leak
}, []);