Hello! Engineering researcher here. I toy with things that are both wildly theoretical (information bounds for algorithms, inference in stochastic dynamical systems, etc.) to things that are fantastically and directly useful (design of photonic structures for LIDAR, (much) better AR/VR lenses, etc.).
While you're possibly right that I might be "building prototypes that aren't quite useful but not quite that novel," I disagree that they're "[not] interesting." In fact, I'd be absolutely surprised if in a few years, much of the applied "engineering" work our lab does (in contrast to the theoretical work) is not in constant use for on-chip photonics and fabrication of optical structures.