It matters how a fictional system "works" exactly as much as the vicissitudes matter to the plot, and to a lesser extent, exactly as much as the actors involved are equipped to understand it. GRRM's winters and their mechanics matter a lot in ASoIaF because they are the looming "problem" to be solved or failed-to-solve, so whether or not the narration "reveals" the underlying scientific rationale, the looming problem will not feel plausible without some internal consistency, even if it's only visible to the author.
You don't have to know the entire economic history of your fantasy world if the "problem" is that magic is weakening and the dragons are dying, unless you intend for the economic system to be either a cause of that problem, a solution to that problem, or a hefty barrier to understanding that problem. It helps to have a working understanding of general economic history - who has what resources, who works with who to acquire those resources, how resources are acquired, who has wealth, pays taxes or tithes or protection fees in coin or grain or cattle or credit-transfers, who protects the movement of wealth, what the consequences are of "lacking resources", what the social safety net is like - obviously, because this helps you populate your world with characters that exist and have basic economic considerations other than how many russet apples and rounds of wax-coated cheese they can fit in their saddlebags.
This helps make power feel palpable in the hands of those with authority, powerlessness feel palpable when interfacing with those who are truly desperate in your fiction-context, and helps raise stakes other than the central ones, which helps develop character conflicts that can be addressed without "solving the whole problem of the book". If every conflict is about dragon-magic, the world will feel one-note and you will struggle to develop characters on any other axis, as they have no other needs or social context than their position on the issue of dragon-magic.
Similarly, geology enriches your setting because it helps establish where your castles can go, where they can't go, etc. It also gives you an "avenue" to either start or end conflicts - if the siege-position is just too unbreachably strong, the aquifer beneath the castle too cool-flowing and the larders too full, it either heightens the competence of the invaders to have taken it, or gives you a plausible reason to have the invaders ultimately turn away. For that, the stone matters, the water matters, and the food source and preservation methods matter.
If the invaders turn away for no reason other than "it took too long", when they have with-some-difficulty taken similar castles before, or take the castle for no other reason than "they were cleverer than the guards", there is no opportunity to mine that interaction for deeper meaning - to show the brilliance of the leader of the invading force, who found and poisoned the aquifer using just the right magic or just the right acid to destroy the defensive fortifications and cave in the west wall, turning the water source into a siege weapon! To show the unique resilience or forethought of the castle-leaders, or their carelessness with their bounty of water.
If you don't examine yours systems carefully and with thought and research and curiosity, you are missing out on not just the "seasoning" that goes on top of a story with a materialist bent, but the material itself. You don't have to do it. But it really helps.
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