Warum ist der Context als ThemeContextType | undefined typisiert?

interface ThemeContextType {
  theme: "light" | "dark";
  toggle: () => void;
}

const ThemeContext = createContext<ThemeContextType | undefined>(undefined);

function useTheme() {
  const ctx = useContext(ThemeContext);
  if (!ctx) throw new Error("useTheme must be used within ThemeProvider");
  return ctx;
}