I mean, you meet a guy on the other continent. And he sends you 50000, idk what tonnes of iron to supply your troops continously for centuries while you might might not even have a port city existing to send him a ship with currency, much lighter trade commodities and treaties once in a while.
THis improves strategic DECISION MAKING so much. Suddenly, you have to make a war with other units to get horses or iron or coal, not magically import from around the world. Suddenly you need a port city to trade by ships with a nation on other island or with which a land route is occupied by another enemy nation. And suddenly, you need a navy, even if just small, just to protect this trade route from barbarians and pillaging. And port blockade=problems with economy, happiness, resources, like in real life. Suddenly you have to make another city, or rethink your placement, just in a spot to reach another AI. There were many reports happinness management is too easy, so guys report having no issue with it no matter how wide they go. Suddenly, happiness might get much more fragile, much more dynamic, cause you would be automatically exluded from trading with half of AIs on a standard game without some cities as trade outposts. SUddenly trade routes deicion must be so much more thought-out, not just extra yields. Limiting anything in a strategy game is such a brilliant move. Suddenly one game will feel more different from subsequent, not just a copy paste like sometimes feel now.
This add so much depth by a single change.
I also consider all other side eeffects like imiting WLTKDs or limiting strategic resources as GOOD. LEt have some wars for lux/strat resources as they were in real life.