I find the existence of number theoretic problems quite puzzling. I wonder what are the implications about the world we can make from them.
In other words, it is an issue between access and existence. Does access need existence? For instance, when X says 'John is charismatic', is 'charisma' like 'neurosis'? Definitely not, and X sees John's charisma. And this charisma doesn't play the causal role.
Maybe we don't know today if there's an application, but notice that proof mentioned using Fourier Transforms to study a set, the same way you might use it to study a radio signal. There was probably a time when people asked what use is FT?