Why hello() .. shouldn't it be main() ??

Answer 1

You can write main() if you want to. For this case, it doesn't matter.

Answer 2

its not compulsory to write it outside of main but what will happen when you want to use this function in your another function like you have created a function in main and it used to get factorial of any number.
You created another function that find value of sine.
You need to use factorial to find sine value. You cannot access function in main. You have to write one function two times. Want to waste time, and call yourself a programmer.

Answer 3

you can do it, but it is not a Good idea. a language is better with better practices.