You're only thinking of the immediate practical applications of the mathematics, not the importance of that work for future work.
Turing's work was so important that he, not Babbage, is generally recognized as the founder of computer science.
One example of the importance of Church's work is Lisp. Church's lambda calculus is the foundation of Lisp, which pioneered nearly all the features of modern programming languages [1]. The designer of Smalltalk, Alan Kay, called it "the greatest single programming language ever designed" [2] and talked about its influence on Smalltalk quite a bit [3].
And your example, the Fourier transform, was itself a mathematical tool long before the first computer was built, and the first published FFT algorithm also dates from the 1930s. [4]
1: http://www.paulgraham.com/diff.html
2: https://www.quora.com/What-did-Alan-Kay-mean-by-Lisp-is-the-...
3: http://worrydream.com/EarlyHistoryOfSmalltalk/
4: https://en.wikipedia.org/wiki/Fast_Fourier_transform#History