Spatial tree data structures like k-D, quadtrees, octrees, and so forth, are very useful for many fields, say computer vision or mesh- and grid-based (adaptive) PDE solvers.
On the topic of that Python thread. I had the pleasure of collaborating with some hydrologists which used the Fortran quadtree implementation I wrote to simulate mass transfer among dispersed particles: Parallelization of particle-mass-transfer algorithms on shared-memory, multi-core CPUs - ScienceDirect