Why we need function in c programming!

Question

using functions we return value.
then we print whether its true or not!

Answer 1

printf is one of function if you dont want any function so what will result be?

Answer 2

functions are for to run the same functionality multiple times. somebody have to write it once, than them or others can use it in other places in the code.
a good example for this is the is_prime(int num) function which returns true if its parameter is a prime number. you can write this function and use it in multiple places or even miltiple programs too.