Saladin Ahmed’s essay on the NPR website argues that the appeal of epic fantasy isn’t in its plots, characters or themes; it’s in its creation of a vividly detailed secondary world.
[A]t its best, work that prioritizes world-building offers pleasures that just can’t be found in other sorts of literature, the joy of traveling to, as Tolkien put it, “a Secondary World which your mind can enter.” The type of immersion that a massive built world provides is unique. It’s an almost physical sense of getting lost somewhere that isn’t home, but which comes to be home. A sense that one is walking, sometimes even dancing, on a tightrope between the fantastic and the mundane. As with the Thousand and One Nights, which so often — and yes, clompingly — mentions things like which vegetables were just bought or who the monarch was at a given time, the modern fantasy novel’s nerdy attendance to world-building gives it a strange mimetic heft not present in, say, fairy tales.
If you accept Ahmed’s argument — and I can see no reason not to — then the usefulness of a fantasy map is immediately obvious. If it’s all about perceiving that secondary world, the map is literally the key to that world. As Ahmed recalls George R. R. Martin telling him, “When college students and hippies started hanging up Lord of the Rings posters, Martin pointed out, ‘It wasn’t the book covers or some artist’s conception of Frodo that went on our walls. It was the map of Middle-earth.’”
Conversely, you could argue that a story that isn’t principally about the world-building doesn’t need the map.