A quarter of a century ago I still produced campaign maps for Dungeons & Dragons by hand, using hex paper and colored crayons. Now that I am preparing a new campaign, I realized that I had better options these days. I now own a color laser printer, and drawing software has come a long way. So I invested in Campaign Cartographer 3, with added Dungeon Designer 3 and Fantasy Floorplans. Not really cheap, and like all CAD programs a bit difficult to learn, but the result is well worth it. Not only was I able to make a map of the island my first adventure will play on, but more importantly I created my first battle map.
4th Edition Dungeons & Dragons in some ways has gone back to its roots of miniature wargames, and those are best played on some sort of a battlemap with a square grid on it. There are quite a lot of options on how to do those. You can take a big piece of paper with squares from a flip chart and draw on it, which is fast and cheap, but not very pretty. You can take use Dungeon Tiles, Flip-Mats, Map Packs, or similar commercial products; but then you are limited to the designs on the product you bought. So if you want both pretty and complete control, using software to draw the battle maps yourself is the way to go. For example I printed mine on 2 by 2 sheets of A4 paper, so now I got a battle map with 16 x 23 1-inch squares, which works best for the usual size of miniatures. And it looks very nice, with a green grass texture, boulders, trees, and even a camp-fire. I can make similar maps with caves, dungeons, and whatever else I want, and they serve both as play surface and visual aid for my players.
The downside of these self-made battle maps is that by the time you designed them, printed them, and glued them together with sticky tape, you spent more time on making the map than the players will spend fighting a battle on it. Thus I'll also work with commercial printed maps, and just design the adventures around them to make them fit the story. And if all else fails, my group is using a laminated blank battle map and white-board markers to draw on it. Nevertheless I love maps and I'm quite happy with the progress that software made in this area.