Results/concepts that also proved useful outside of their "home areas" There are some results/concepts in TCS which are used in areas other than the "home area" where they emerged. For example, NP-completeness has complexity theory as its home area, but it is also used in many papers that are not about complexity theory. This is an indication of the general usefulness and importance of the concept. 

Similarly, network flow algorithms, shortest path algorithms etc.,  have combinatorial optimization as their home area, but are also used in many papers that are not specifically about combinatorial optimization. Graphs have graph theory as their home area, but are used in many other fields, as well.

Question: What are some other, not so obvious or not so well known examples of methods/concepts that also proved useful outside of their home areas?
One thing that immediately comes to mind is that graph k-coloring is used in compilers for register allocation. An instance of register allocation for k registers is solved by reducing into an instance of k-coloring. See, for example, section 8.8 of Aho, Lam, Sethi and Ullman (i.e. Dragon Book).