What is useId used for in React?

Direct answer

useId generates a stable identifier that is useful for connecting accessibility attributes such as a label to an input across client and server rendering.

PasswordField.jsx

function PasswordField() {
  const hintId = useId();

  return (
    <>
      <input
        type="password"
        aria-describedby={hintId}
      />
      <p id={hintId}>Use at least 12 characters.</p>
    </>
  );
}

The main use case is accessibility relationships and generated IDs that need to be safe across React's rendering model.