Kleene's theorem that finite automata (specifically, nondeterministic) are expressively equivalent to regular expressions seems to be a powerful and not immediately obvious tool for untangling the state diagrams of NFAs (by translating an NFA to a regular expression and back).
Has this been found to have applications within pure mathematics? I could imagine it having something interesting to say, for instance, about the generation of monoids.
This has been difficult to search for, on account of so many theorems being named after Kleene.