you know, sometimes i wonder if the designers put the knowledge skills in the game for like, a purpose or if they and their bonuses are just there as decoration on the character sheet
(a similar thought is had about social skills)
I cannot speak to what the designers themselves intended, at least for most editions.
But I can say, at least for me, I prefer it when they are much more than that. Which is one of the reasons why 4e's skill philosophy (separately from its design) is by far my favorite of any edition.
Because in 4e, skills are
chonky.* If you are trained in Arcana, you aren't just able to identify a spell being cast. You know the academic (but not necessarily practical) details of ritual magic. You can identify supernatural effects, even if you don't necessarily practice that type of magic yourself (e.g. you can tell if something is divine, but not necessarily its source, while someone trained in Religion could identify the source, but not necessarily the details of how it works).
I find that History is the best litmus test skill here. In games where, as you say, knowledge skills are a "decoration on the character sheet", you can generally tell this fairly quickly because the skill won't have any application outside of descriptive fluff text. Conversely, when it does matter, History becomes an incredibly useful skill, because it covers much more than just a dry chronological accounting of events. History, for example, is the skill for knowing
battle strategies, by which I mean grand-strategy, "literally conducting a military campaign" type stuff--which means you can use it to survey the enemy's disposition of forces to try to predict what they're doing. Paired with good maps, it's useful for triangulating locations of historical places. Observing an item, it's useful for distinguishing forgeries from genuine articles, and for connecting artistic styles to particular cultures or (for more recent works) particular creators. And it has subtle social uses and implications, as it takes on roles like etiquette rules for different social groups, or myths and legends of a particular culture to indicate the values or norms of said culture.
Chonky skills make the game, IMO and IME,
much more engaging and exciting, because it means players start thinking about things by asking questions, rather than only thinking in terms of how to survive (and, as a consequence, how to maximize character performance to ensure survival). "Thin" skills, skills that
only have narrow, predefined uses and can't be used for anything
not pre-defined, instead very much discourage creativity and exploratory choices, because they mean most creative actions are at best a waste of time and often actually harmful. This results in both character homogenization (everyone is good at Perception, Athletics/Acrobatics, and Intimidate, for example) and behavior flattening, where players will simply
not care about other stuff--it has no value beyond descriptive text, and if the descriptive text were useful, it wouldn't be locked behind a single skill.
*In theory they should be so in 5e as well, but I've never seen it play out that way, and people don't describe running them that way either. I don't know why. The text gives no reason for doing so and multiple reasons for not doing so...but people do it anyway. It's a 3e-ism that actually
defies the text and I genuinely can't explain why this happens, but it's a pattern I and others have seen.
