À quoi sert la fonction assertNever ?

function assertNever(value: never): never {
  throw new Error(`Unexpected value: ${value}`);
}

type Shape = "circle" | "square";

function area(shape: Shape) {
  switch (shape) {
    case "circle": return Math.PI;
    case "square": return 1;
    default: return assertNever(shape);
  }
}