Grandt Line parts for sale tend to show up once you get into scratchbuilding or refining structures beyond what comes in the box. Their doors, windows, and small castings are used across HO, N, O, and narrow gauge projects where proportions and detail actually matter.
These are the kinds of parts that let you control how a building looks instead of working around pre-made kits. You can find more in model trains when tying structures into a full layout.