It is purely academic... or is it?

G. H. Hardy's autobiography was titled "A Mathematician's Apology" because he, somewhat jokingly, wanted to apologize for a life of pure math... totally academic and completely useless.

Then a few years after he passed away, his math was key for The Manhattan Project to build the first successful nuclear bomb. His math lead to the device that changed the world and affected every aspect of human life for a century.

It's only "useless" until someone finds a "use".

P.S. The book documents his time with Ramanujan. If you liked the film "The Man Who Knew Infinity", you should read Apology