Having a map for your adventure adds a whole new dimension to your game. Whether it is a map of your world, of the local geography, of a city or town or even a map of a cave system or dungeon, providing a visual representation of your game provides better detail than words ever could. It creates questions and curiosity for the players, gives them a sense of scale and a sense of orientation in your world.
The map doesn’t have to be highly detailed or polished. It could be as simple as pencil-sketches on looseleaf paper :
Or more ornately detailed, as in this example from E.G.’s game :
Or one of EG’s hand-drawn maps, which I re-rendered in MS Paint :
Or entirely composed with MS Paint :
Or a graph-paper dungeon grid made in Excel, copied into MS Paint, saved to .JPEG format and printed for use :
Or a real-world Google satellite map co-opted for use in your game :
The entire map doesn’t have to be revealed all at once; as DM you could own a master copy and provide a working map to your players, which would change as they pushed back the “fog of war” in their exploration.
Your choice of landmarks and geography can set the stage for later acts in your story, foreshadowing encounters or, as need be, providing containment for low-level characters until they are skilled or equipped enough to advance.
Ironic place names can add a sense of humour to your game (a region called Hallowed Hills that is swarming with zombies). Forbidding geographical names (Black Death Marsh) can create a sense of unease in the players if a mission has them scouting nearby.
The level of detail is up to you – understanding, of course, that too much detail might be a waste of your time or it might constrain your ability to improvise during a game, or to change your story’s direction later on.
In a dungeon or cave system where there are multiple paths, it is possible that players might not send their characters down alternate pathways or into many of the rooms you might have spent a lot of time constructing and populating with nuances like traps and valuables. Or characters may tunnel, teleport, pass through walls, or otherwise defeat a certain path you have charted if they are of a mind to follow a straight and simple path to a destination.
Really consider providing at least a geographical map of your realm, even if you do not plan to formally map out towns and cities and encounter spots in great detail. Having a map takes a lot of ambiguity out of your realm, gives your game a sense of setting and personality, and can work to your benefit in later adventures and story arcs.