Still i wonder why: this because RPG are the paramount of imagination to my eyes! Being games where ppl sit around the table and imagine collectively a story to live together, this is the top of the effort in immagination... So why the presence of a "little push" like an artwork could be?
Fundamentally, to give framing. To set boundaries -- be that because you intend to adhere to them, or because you want to know when you are breaking them. You can't color within the lines without lines, but you also can't color outside the lines without lines (and, while you might not want to color within the lines, you may still constrain yourself to coloring on the page and not halfway across the kitchen table as well).
Let's take AD&D (1E and 2E) core books as examples. Both have covers which depict rather epic individuals (wizards wielding magic, dungeon-crawlers removing torso-sized jewel-eyes from giant statues, etc.), but once you look inside, the have decidedly different tones of artwork.
- 1E includes a lot more depictions of incredibly scared-looking adventurers meekly sneaking through dungeons like the next turn probably will be their doom. Also lots more comedic depictions of characters slipping on banana peels or sneaking past giant rats while wearing mouse costumes saying 'this had better work.' Also lots of depictions of the world of 1E being Conan/Fafhrd&Grey Mouser-esque shadowy back alleys full of cutpurses and seedy taverns.
- 2E instead includes lots of depictions of epic characters kicking in dungeon doors and laying waste to their enemies. Adventuring parties in power-poses. Individual mages standing there crackling with energy. Domestic/urban scenes hew more towards friendly inns with roaring hearthfires, market scenes, and so forth. Excluding an unfortunate penchant for fainted-women-in-peril* art, most of the people in the scenes are being victorious. And often epically so (fighting the truly powerful beasts of the game). The most grounded picture I can think of from 2e is a party of adventurers with a defeated dragon hanging from a tree, and the only constrain there is that it is clearly a very small dragon. *and these are typically unarmored women most-likely not part of the party so much as someone the party has to rescue.
These two different framings support (or honestly reflect, since the change happened in late 1E) that 1E and 2E had a change in how the game was generally perceived (and oftentimes played. And this is with rulesets that were* largely the same.
*I'm going to get flack from purists of both editions, but you certainly at least know what I mean.
Pls consider the example you gave with the calymore definition: why is necessary to provide details of the shape and the structure this sword has? ...and i do not want to consider its artwork in the picture... Wouldn't be better (by far to my eyes!) to leave an unprecise description that it is a large sword and the game mechanics info (namely damage and speed for instance)? In this way everybody is free to imagine within his game what kind of claymore he likes better...
If someone is wielding a basket-hilted claymore one-handed, it is (/can be) a grounded fantasy. If someone is wielding one of the big two-hander claymores one-handed, it is some level of over-the-top fantasy. Both styles of game are fine, acceptable, even great; but in general everyone should be clear on which of the two types of games you are playing in (before someone does something like have their character try to jump a 12-meter crevasse or something).
Do you understand where i want to get?
It would be like having a movie played while you are reading the book... Artworks would set "constraints" to your immagination... (How many times we have heard that the book is better than the movie IF you have read it before watching it?)
Books often take several chapters (and usually a non-trivial portion of the wordcount ongoing throughout the story) establishing these same constraints specifically because they can't use visuals the same way that a movie or book full of art can. Tolkien spends huge swaths of text focusing on travel and roads and towns and cottages to establish that the main adventure of each of his two main works are exceptional departures from a very grounded pastoral lifestyle that the protagonists normally have, and that the primary purpose of the adventure (one dominated by difficult travel punctuated by a few moments of mortal terror) is to be able to get back to that lovely adventure-free lifestyle. In the Conan stories, Howard spends a huge amount of time describing the towns, the places, the castles, alleyways, the smells, peoples' gaits, their eyes. All this to ground Conan in his world -- a very low fantasy world where even the greatest warrior in all the lands occasionally gets conked over the head and wakes up helpless in a cell.
All of these things are things that a game
can leave up to those playing them (and some of them do, to a lessor or greater extent). However, then the group had better all come to a consensus on some of the scope and tone and scale of their fantasy (and, IMO, it is better that the game have tools which help facilitate this). Either way, the primary goal is that the game people want to play and the ruleset work well together, and if the games' artwork communicate what the game will be good for, IMO that is great.