Need of privatizing assignment operator in a Singleton class

Can someone justify the need of privatizing the assignment operator in a Singleton class implementation?

What problem does it solve by making Singleton& operator=(Singleton const&); private?

class Singleton {
public:
  static Singleton& Instance() {
    static Singleton theSingleton;
    return theSingleton;
  }

private:
  Singleton(); // ctor hidden
  Singleton(Singleton const&); // copy ctor hidden
  Singleton& operator=(Singleton const&); // assign op. hidden
  ~Singleton(); // dtor hidden
};