Art has several roles...
- To Inspire
- To Inform on the content - examples, maps, and flowcharts
- To mark units of the book (Both graphics layout and chapter header and chapter art)
- To fill space
- To provide tools - counters, templates, forms
CT LBB's - both editions, are almost entirely information or tools.
D&D art in the LBB/LWBs was largely inspirational, with some for information, and some to fill space. There are more pieces of art in V1 than in Traveller LBBs 1-3 combined. (And oh, how low the quality. Arneson's sticks out as decent.)
CT expands immensely in TTB, where a lot of inspirational art is added, and a lot of small informational art is added (ships, weapons). D&D was more art.
Traveller was written like a textbook. D&D was written like a fanzine.
D&D needed the art heavy design - it was dealing with stuff people didn't often have strong cohesive shared visuals for.
Traveller was blessed by having several very popular shared visualizations to draw from - Star Trek, Star Wars, BSG.... The core had no non-humans, so no informational art needed for them. Most of the weapons were easily looked up straight away. Leaving the ships out of the LBB core appears to have been a space saver... but also, I'm not sure that concrete ideas for their look existed when CT-77 was released...
Later editions fell into a trap - art for the sake of art, because everyone else has lots of art. But everyone else NEEDED art, for their worlds were not readily visualized.
Now, the current Star Wars - the art sets off chapters (big, 1-2 page spreads), illustrates nearby text, and inspires by visualizing the setting material. Almost all of it has functional roles. TTB, some of it is just there to be "pretty". (EG: "In case of fire, break glass" - TTB 51 - seems to be just humorous padding...)