Statistics and probability (up to an undergraduate level) are necessary if you want to work in machine learning. Category theory is nice (but not necessary) if you want to understand the underpinnings of Haskell.

What I think really makes math generally useful, though, is that, at its root, mathematics is a formalization and generalization of problem solving. That is what formal systems and computability theory, for example, are actually about: how, in general, problems are solved.