I’ve heard it said that you can’t do anything in a game without a map, but I respectfully disagree. Let’s start with a couple of questions and my answers.
DO YOU NEED A MAP?
Not really. You can tell a story without a map; books and movies do it all the time. You can have a dungeon without a map; take a look at the Beasts & Barbarians free adventure The Carnival of Nal Sagath.
WHAT ARE MAPS FOR, THEN?
First and foremost, a map exists to offer the players a clear view of their choices. Should they go through the swamps where the Rodents Of Unusual Size are said to live? Where should their lone scoutship make its stand against the oncoming fleet? But you don’t absolutely have to offer them a choice; a lot of adventures are linear, and how many scenarios do you actually have worked out for tomorrow night’s game, anyway? If you do offer them a choice, you can easily do so without a map; remember, just because the characters know where things are doesn’t mean the players do. It helps if the number of choices is limited, as without a map the players have to hold the salient features in their mind’s eye, and so do you as the GM.
Second, a map is a handy visual hook for the game, which draws the players in and helps invest them in the setting. The style of the map conveys subliminally the kind of world it represents.
Third, it assists the GM in maintaining a consistent, persistent world. ("Wait a minute, last time you said that tower was three days travel east? Has it moved?") It’s not the only way to do that, though; narrative speech works just fine, especially if you’re vague about distance and travel time. (“Last time, you were travelling upriver towards Stonebridge. You now arrive at the gates of that town.”)
As ever, the key question is: What are you trying to achieve?
If you want an instant overview of a setting, a good map is useful. (Ones with little pictures of what you might find in each area are good.) If the characters are in a small and restricted environment, a map isn’t necessary. If the setting is one where all locations are effectively equidistant from all other locations (like the planets in Stargate: SG-1), you don’t need a map.
If you want the players to have a wide range of choices without tying the session in knots describing them, a good map is useful; in fact, when I’ve done this without a map, the players instinctively try to draw one. If the plot is linear, you don’t need a map.