About returning a reference

Hi, I am going through Jesse Liberty's "Teach Yourself C++ in 21 Days" and found the following program with an example of overloaded assignment operator:

#include <iostream>
using namespace std;

class CAT
      int GetAge() const {return *catAge;}
      int GetWeight() const {return *catWeight;}
      void SetAge(int age) {*catAge = age;}
      CAT &   operator= (const CAT &);

      int *catAge;
      int *catWeight;

      catAge = new int;
      catWeight = new int;
      *catAge = 5;
      *catWeight = 8;

      delete catAge;
      catAge = 0;
      delete catWeight;
      catWeight = 0;

CAT &  CAT::operator=(const CAT & rhs)
      if (this == &rhs)
            return *this;
            *catAge = rhs.GetAge();
            *catWeight = rhs.GetWeight();
            return *this;

int main()
      CAT *Cat1 = new CAT;
      cout << "Cat1's age: " << Cat1->GetAge() << endl;
      cout << "Setting Cat1's age to 6..." << endl;
      CAT *Cat2 = new CAT;
      cout << "Cat2's age: " << Cat2->GetAge() << endl;
      cout << "Now setting Cat2 equal to Cat1... \n";
      Cat2 = Cat1;
      cout << "Cat2's age now: " << Cat2->GetAge() << endl;
        return 0;

I have a simple question: why does the overloaded assignment operator return a reference to a CAT object rather than a CAT object? Does it make any difference if you change it to return a CAT object instead? I tried changing it and couldn't see any difference in the output; after all all it should do is make a "shallow copy" of Cat1 and point it to the same area in the free store.
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It is necessary for writing assignments like:

Cat1 = Cat2 = Cat3;

It is executed by the following way:

Cat1 = (Cat2 = Cat3);

This means, Cat3 value is assigned to Cat2; result of this assignment (Cat2&) is assigned to Cat1.

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For example, stream << operators always return stream reference:
ostream& operator<<(int).
If operator << returns void, it is possible to write:
cout << n;
but line
cout << n << endl
is not compiled.
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