Riemann's functional equation satisfies the relationship $$f(x)=f(1-x)$$ which in part has earned the function that satisfies this its own name, known as the Riemann Xi function. A lot of functions that solve simple functional relationships have widely practical uses, like radicals, exponents, logarithms and the gamma function which are all used in every or nearly every field of science.
Is there any practical application for this function beyond number theory and other less common topics? Maybe in algebra or calculus? Maybe in physics that isn't limited to 6 or 11 dimensions?