Finding large prime numbers is an essential part of the mechanisms involved in Internet security and privacy. And not just prime numbers, sometimes you need to perform various calculations on numbers that are specifically not prime. There are also uses of methods you have familiarized yourself with during your childhood, such as greatest common divisor (GCD) and least common multiple (LCM).
This is a companion discussion topic for the original entry at https://toph.co/tutorials/eulers-totient-function