|Image used with permission of Jack Monahan|
However, in many cases books are not entirely text. An occasional illustration here and there may exist, providing a vague but definite framework to the series - what a character looks like, what the environment looks like, and so on. One non-book example comes from Half Life's Gordon Freeman, who is never seen (there's one picture of him but it's not identified as being him) in the game, allowing players to imagine their own ideas of appearance. This shifted with the release of Half-Life's Game of the Year edition, which put him fairly flagrantly on the front cover (compare with the original cover). This simple change turned Gordon from a faceless player-avatar (with an established background) to a definitive, recognizable individual who just happened to never talk.
The point, then, is this: even if something is largely an "imagination-based" medium, like a book or a pen-and-paper RPG, the visual design and illustration of the series establishes a framework for people to think about the game. Obviously this has positive and negative effects. It's positive because it's evocative and helps to establish what the setting is supposed to be like, giving definitive shape to characters, places, and events. It's potentially negative because it can curtail the role of imagination. This can be especially problematic with RPG sourcebooks, which are meant to represent a wider range of concepts and settings beyond what the book sets out.
This means that some very subtle differences in artistic tone and direction can totally shift the nature of a text-based medium. While these things tend to be regarded as less important than things like "good writing" or "solid rulesets", they're still pretty important in establishing a mood as long as the art styles within a given product are relatively consistent. A shift in art can make or break a mood, and while each art style is going to have its own fans, it's also going to be a different group of fans.
Compare the art of Karl Kopinski to the work of Mike "Daarken" Lim. They've both done work on Warhammer Fantasy and Warhammer 40k, but Kopinski represents the "old school" and Lim represents the newer editions. The thing to notice is that while they depict the same things, and have a lot of similar themes and tones - dramatic battles with lots of emotion and conflict - simple things like color composition and lighting make them very different in tone. It's a markedly different style, and as such has different associations in terms of the tangibility and believability of the images.
This is a difference that results even when the things being depicted are actually the same. Now let's ramp it up by comparing D&D's different editions, from AD&D to 2nd Edition to 3rd Edition to 4th Edition. You could say that the artwork has improved in technical terms, but it has also changed thematically as well. 1st Edition's artwork was very basic and medieval-derived. 2nd Edition was obviously more fanciful, but still fairly realistic - it was exaggerated and heroic, but still based in reality. 3rd Edition became more about the glossiness and stylishness of the design, going for a more "punk" approach. 4th Edition was similar to 3rd, but took the general "glossy" feel even further, making things feel more cartoonish (at least to me).
No matter which of these editions and styles you prefer, there is an obvious difference between them. The style reflects on the gameplay, because those images are there to provide a framework for the player's understanding of the people, creatures, equipment, and other things within the universe. Even if everything else is the same, the art style is going to play a major role when it comes to a reader's baseline understanding of the universe.
I could talk about a lot of other franchises that shifted art design (Warcraft would be the biggest one), but the point I'm trying to make here is that even when the design is not explicitly connected to the product, the art choices still influence how people think about the game. The same is true for books - something as simple as a chapter illustration or cover art can provide a reference point. If it's turned into a movie, it can be even more severe, as now every part of the universe is given a distinct visual appearance.
TVTropes uses a term called Inkstain Adaptation, and while not entirely the same, it gets across a lot of what I'm trying to illustrate. If you have a vague concept, people can do with it what they like. If you have a definite concept, it's going to color their viewpoint on the franchise as a whole. Therefore, establishing that definite concept is a major concern when it comes to creating a new franchise, or expanding an existing one.
To Sum Up:
1) Visual design and illustrations can serve as points of reference even in a heavily text-based material.
2) The audience is going to extrapolate from the images they see what the rest of the universe looks like based on the style and composition of the illustrations.
3) Changing the style and illustrations can have much more far-reaching consequences than simply being "new art" because of this.