Why do we use return 0;

Answer 1

Short answer: just put it and forget about it.

Long answer: if you launch your program with another program, the host program can use the return value of your program to do things differently. A return value of 0 means no error, a non-zero value means any error of your choosing. That feature is seldom used, so don't mind that and just put return 0.

Answer 2

To add onto Zen's answer, most compilers will add it automatically if it's not there when the code is compiled.

Answer 3

So as to end the compilation of the programe