Currently writing software for electrical engineers. One example that comes to mind is graph theory. I use it frequently to answer questions about the data I'm looking at.

Imagine an electrical distribution network where there are 3 phases (A/B/C) and an overhead line can contain any combination of those phases. These networks can be modeled as a graph with nodes connected by links. The links do not have a direction, but they do have a phase. If you want to know what the phasing is at any point in the network, the way to figure that out is to use graph algorithms.