Have you ever read a high fantasy book (the fantasy books written in a world other than our own) and desperately wished it had a map in it? You read all the places the character goes and just can't picture them in your mind on the map. You get confused and your mind begins to twist into knots. You can't keep everything straight. All of a sudden, the twisted mind and struggle to understand becomes the only thing you can see about the book. There are no longer beautifully described scenes and wonderfully extravagant plotlines and characters you fall in love with.
There is only a confusing setting that you cannot wrap your head around.
That does not mean every "high fantasy" novel needs a map. I have read plenty of novels set in other worlds where I do not get confused. Generally, these are the books where the character doesn't move around too much and, in particular, doesn't move around much outside of the region or realm they live in. Elaborate names, histories, geography, and governments make maps even more necessary as well as characters traveling to and fro between different kingdoms, realms, regions, etc. If you are going to name mountains and rivers, oceans, and gorges all within a kingdom, all that a character visits within the story, readers will appreciate a map to visualize the places.
But the importance of maps goes beyond the reader's perception. In fact, if you are an author working in an elaborate world built for your novel, in which your character is going to move around in... why do you not have a map?
I know that the maps I have built for myself are not final drafts. They may be beautiful to me but they are also hand-drawn and imperfect. In order to make it into a book, they would need to be copied and edited, and helped along by professionals. The exact details of the map are not what is important to my writing. Instead, it is so much else.
I use my maps to get an idea of where my character is in the world and where they need to be. I use them to understand how long it might take them to get to a certain place. By horseback. By carriage. By foot. I use them to mark where cities are and where the main palaces are. I also mark where my characters grew up as that is important to my story (though it may not be important to yours). Bodies of water, mountains, and large rivers are marked on my map as well as forests (though not woods within the kingdoms' boundaries). When I decide on adding a river or other geographical addition to my writing, I add it on to the map because I have 7 books I am writing and thus, 7 characters to keep track of. If someone else crosses that spot, they had better also run into that geographical barrier.
That all sounds great for me but it did take me a while to create the maps (especially since I had to scrap them when I decided to go with a different kingdom than I had originally planned on). So where do you even begin?
I have seen a number of Pinterest posts that suggest throwing a number of dice at a piece of paper and letting the dice decide how the map is drawn. Many even suggest letting the numbers on the dice decide how the population is spread out.
I personally do not like leaving my maps to chance.
When I create a map, I like to first determine the things that need to go into that map. This is not "rivers" "mountains" and "cities" but rather it is determining the answers to questions like "how many kingdoms are there?" "what are their names?" "how big are they?" "what resources do they depend on and therefore what do they need to be close to (ocean, forest, mountains, other kingdoms, etc.)?"
From there, I can begin to think about the shapes of my kingdoms, where I want them to be laid out on the map, and where the oceans/lakes and forests should be laid out around them. I think about the politics, geography, and relations each kingdom has with one another as I do this and plan accordingly. It is only after I have drawn in the lines of the kingdoms that I begin to add in mountains, rivers, and other geographical elements, thinking logically about where they need to be placed in order for the kingdoms to survive and thrive.
Finally, I can add in things like Palaces, childhood homes, and cities. As I do this step, it is important to think about distance. If a character stumbles into something while running from their home or attends a ball at the Palace and returns home at midnight, their homes must be set close enough to the other places so that it makes sense for them to be able to get to where they need to be within a certain amount of time. Cities with larger populations shouldn't be out in the desert away from water sources and Palaces are likely to be surrounded by cities and towns that want to be close to the source of power.
I have two maps (which you might have been able to tell from the fact that I have used the word "maps" as opposed to "map" all this time). On map shows my four kingdoms as well as the Isle of Meermin in the time that most of the books are set in. The second shows the kingdoms 100 years prior when The Sluirmeren Kingdom reigned the Ooit Dor Lands before it fell after the 100-year sleep. That probably makes no sense to you (given that it is really just a sentence of nonsense when it isn't in the context of the series) but the real point of saying that is to point out that I have two maps: one to show me where one character roams and one to show where the rest of the books are set. The maps account for time difference—the borders are different (the forest is more taken over by the kingdoms, and the kingdoms' border lines slightly changed), the population has increased and cities have grown. The significant feature, though, the thing that should be pointed out, is that it makes logical sense.
Your story may be fictional. The world you write in maybe fantastical. But if you chose to ignore all logic and sense, you will lose readers because of it. Even fantasy books need order and logic. Even magic systems need rules. Even lawless criminals and terrible villains fight for justice, flawed as their sense of it may be. Lose your recognition of that and your fictional novel may fall into chaos.