Art in the TTRPG industry is oftentimes relegated merely to decoration, it's main purpose drawing attention to the text inside a book. It makes sense: just by the fact that we're approaching art in such a broad way reveals how, we designers, don't take art too seriously. That's why I'm gonna get specific, I will not talk about "art", I will talk about Illustration.

And Illustration is game design.

Illustration as a discipline has the stated goal of complementing an accompanying medium, usually text, to better convey its themes. The act of illustrating a game gives you the chance to present information to the reader from different angles, helping economize text glut and deepen its subjects.

TTRPG books are expository texts in nature: describing game rules, setting, fictional situations, etc. At best, not using illustration to its full potential misses out on the opportunity to improve your expository writing. At worst, bad communication in Illustration detracts from the writing: how many times have we seen art that show a situation that can't be replicated through the game rules? How often an illustration presents information that contradicts the text? This muddles a game's exposition, ending up on readers building false expectations, getting confused, and, finally, ignoring the art altogether. 

Like any tool, Illustration can be used to great effect in TTRPGs, adding a visual dimension to your game design. 

Taking Noctis Labyrinth, if an encounter describes three quetzalcoatlus swooping down to attack, the art should illustrate that scene. If a creature description says there's a 1 in 3 chances a desert demon carries a thunderstaff, the companion piece should show that. By expanding the use of illustration beyond decoration, I integrated that element into the book's information economy, creating a virtuous cycle between art and text that facilitates exposition to the reader. 

A hypothetical GM could describe these encounters at a glance just by putting into words what they see on the page, which would then buy them time for a more detail read of the encounter as it unfolds at the table. Compare that to the traditional method of writing a long, dense paragraph, describing characters, measurements, positioning, distances, area effects, etc. for the GM to then have to read, internalize, and decodify to the table.

Those fictional elements could be easily offloaded to scene or character art, maps and diagrams—which are illustrations after all—, or even comics! (but we'll get into sequential art later). Art shouldn't be only used to break text and managing pace of reading. As any other space at the page, it is meant to be used, seized as an opportunity to say something about the game.

Illustration is also a great tool for worldbuilding, working as scaffolding the players can use to construct the fictional space they explore. But art and worldbuilding are subjects we'll cover another time.