For me, a map serves several functions, whether it is to be used for gaming, or for fiction. Both share the same fundamental purpose of defining worldspace. A map also serves to foster creativity and enrich the amount of detail available to the writer/gamemaster. A visual representation allows the creator to quickly glance at the map, pull some features or names of cities from there that weren't being used, and add to the detail of the world. So in essence, it becomes a resource of unlimited and increasing detail that new stories can be written about and integrated with those that already exist. The more elaborate it becomes, the more realistic it becomes and the easier it becomes to generate new ideas as your frame work solidifies.
The more detail a map has, the more the worldspace is defined, and the singular question pushes to the fore: Why? Why do things exist the way they do in this map?
This leads to another subtle feature of the map that writers take advantage of and may not even realize it: Misinformation. Yeah, if the map has only four or five markings of note, chances are, the reader (or player) will be able to figure out which of those markings are used in the book/game. Put four or five hundred on that same map, and now, the reader is unsure of what will be revealed within the context of the story/game, and the things that aren't revealed whet the appetite for more. Suddenly, for the reader, the world has more "gravity" to it.
Moreover, historically-accurate maps were usually crammed with notes on various areas, and were meant to be as information dense as possible. Especially some of the Spanish maps of the Caribbean. Not only were wrecks/sunken ships marked, there was usually dates given and an accompanying description. Same for trade routes.
Not only that, map creation allows me to utilize calligraphy, and since I write by hand for the most part (or have in the past), my hand-writing has become pretty good, too. I try to go as far as possible with each map, and cram as much information into it as possible, even if I ultimately don't end up using everything, it's still there to be used in the future.
One of my goals is to create a fantasy world, and then open it up like crowd-sourcing and just let people run with it, similar to how Skyrim modders will build a mod framework that allows players to create new stories from. Only, instead of gaming, this would be literary.