What is the point of writing prototype for function?

Question

https://code.sololearn.com/c0IavBNtMZBx/?ref=app

Answer 1

i haven't use it before, i believe it's the style of the coder, some like it that way though

Answer 2

Wow great answers but some advance things i cant able to understand because I am new to c, if you have time please give some material or link to study those things