void main() = int main() return0 ?

Answer 1

Well, the answer is, this is the same for a small not important programs (just for learnig coding).
But the large important programs would return value from main(). But not allways a formal 0 value, but sometimes another value. This is a decison of autor this program, how to plane, code, and describe in a program documentation and help. Comonly, return 0 means a program ended well, another value means some problems when program running..

Answer 2

Some compiler will require that your main function return the value 0 to indicate that the program ran successfully.