In a recent conversation with a friend, I was discussing the fact that out of all of the fields of math, number theory seems to be among those that apply ideas from a large number of different fields. For example, modern results in number theory incorporate complex analysis, algebraic geometry, lots of algebra, etc. In this sense, I see number theory as a more "applied" field (not in the usual sense of applied math), than say, something like algebra, hard analysis, algebraic geometry, or logic/set theory.
I was curious to what extent this can go the other way: in particular, what are some examples of theorems in other fields of pure math where a modern result in number theory was an important part of the proof?
Note: I'm not talking about elementary number theory. Of course, basic properties of the natural numbers are ubiquitous, and things like modular arithmetic, prime factorizations, the Chinese Remainder Theorem, etc. are indispensable in other fields. I also am looking for examples in other parts of pure math, not something like cryptography or computer science.