Why i am writing return 0, why not write getch ????

Answer 1

I'm just going to assume that you were taught to write pre-standard C++ (which contains void main, conio.h, getch, etc) and just in case it is true:
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Answer 2

It's just how C++ relays the exit status to your OS. 0 means it exited successfully, and 1 means it failed. This is good for being able to properly display errors as the program terminates.