Today I reverse-engineered a puzzle; I designed a puzzle to get to an answer that already existed. I had a code for something, and I wanted (for my own reasons) to give it to someone else in the form of a puzzle. I had to figure out a math puzzle that would lead to that number, and only that number, while still being interesting to solve.
Is this practical? Of course not. But the answer to the question of “when will we ever use this in real life” is: to have fun, same as anything!
The Opportunity Machine 