When C-3PO was being shiped through Tunesian customs, the beurocrats got tripped up on a certain entry in the customs manifest - "assorted greebles". The box it refered to was full of miscelaneous costume parts, meant to be affixed to the exterior of the robot. "What's a greeble?" they asked. "Oh, uh, it's something that looks cool, but doesn't actually do anything."
The term was invented at Industrial Light and Magic for exactly these kinds of non-functional bits - items attached to costumes and shooting models which serve no function but are visually interesting.
The original Star Destroyer, as seen in the opening scene of A New Hope, was more than eight feet long, and was covered with these greebles. The long, slow pan across the huge ship, with thousands of pipes, wires, portholes, and other visually interesting material, gave the film a sense of scale and wonder. It didn't look like a model of a ship - it looked like a whole ship, and it looked huge.
The effect was carried off by affixing thousands of spare parts from a dozen different modeling kits to the outside of the ship. Sherman Tank models, Spitfires, and steam-era locamotives were repurposed from thier original design and affixed by the score to the plywood Star Destroyer, until every square inch of the ship had its own unique textured surface.
There was a term for this already - kitbashing. Model companies might only make kits for certain kinds of planes, trains, and automobiles, but by combining parts from multiple kits, or crafting your own, a kit for one kind of vehicle might be repurposed and built into another kind. ILM replicated this impulse, but on a huge scale, and established an entire new style and understanding of spaceship movies.
Greebling has continued since, though in modern times it is typically done by computer alogrithm, so that an animator need not hand-place each textural item.
What I find interesting about greebling as a practice is that it is about the illusion of scale, the illusion of complexity. The human eye and the human mind can be fooled, and can see paterns where none exist. Later technical manuals and novels within the Star Wars universe codified each piece of the Star Destroyer exterior, assigning meaning and order to each of the model-parts. But when it was being built, no such meaning existed - the modelers just kept gluing on pieces until they arrived at a ship that was visually interesting and showed up well on film.
They didn't need to write specifications for every subsystem a Star Destroyer might need, or even think about what systems it might embody beyond having engines, lasers, and a bridge. They just had to glue parts on until it looked good.
I've been making role-playing game terrain recently, and have thought a lot about this Greebling. The maps are at a 5ft to 1in scale, which gives cause to have a whole lot of detail. At that resolution, individual leaves are visible on plants, and finger-sized rocks are distinct from one another.
What I'm finding in the drawing of these maps, and thier digital layout, is that the same process that worked for the ILM folks in the early 70s still works - kitbashing. I'll draw a few different kinds of spirals, or tree parts, or ground dirt, and then bash them together and tile them over and over. A few rotations and scalings, and just two or three different varieties of texture can manifest as a robust-looking set of "unique" items. In the above drawing, the spiraling roots of the trees are just two or three different root-spirals, stamped around with varying density and pattern. The ferns at the bottom left are a single image, rotated and layered, with some shadowing applied later.
It makes the process interesting, since each item I draw is not so much a unique form that will carry through to the final map, but rather an "asset" or individual part, which I might duplicate, manipulate, and use over and over.