why do we have to use 'return 0' ?

In C and C++ programs the main function is of type int and therefore it should return an integer value. The return value of the main function is considered the "Exit Status" of the application.

See : http://en.wikipedia.org/wiki/Exit_status[^]

On most operating systems returning 0 is a success status like saying "The program worked fine". In C++ it is optional to type "return 0;" at the end of the main function and the compiler includes it automatically.

In stdlib.h the macros EXIT_SUCCESS and EXIT_FAILURE are defined like this :

#define EXIT_SUCCESS    0
#define EXIT_FAILURE    1

These 2 macros can be used as the argument to the exit function declared in stdlib.h and they can also be used as the return value of the main function.

the return value is useful to check the status when the application exit.
return 0 means no error.

The return value can be used outside the program by the caller program, script or command. Provided two programs:
the program f1

int main(){return 0;}

and respectively f2

int main(){return 1;}

If you call them like this, you get this output f1 success

$ ./f1 && echo f1 success || echo f1 failure

When you call f2 in similar way the output is f2 failure

$ ./f2 && echo f2 success || echo f2 failure

Above chaining is possible because of the return code, also exit code.
Invoke as yourprogram && otherprogram if you want otherprogram to be called when yourprogram in case it finished successfully. Or call as yourprogram && otherprogram if you want otherprogram to be called when of failure of yourprogram