Well. First, I have to say why I produce such detailed maps.
I think the map is crucial for the DM. Looking at a good map can suggest more details to the DM when he makes the description of the room. Looking at a bare 5x5 room with a door sketched over a wall cannot suggest anything and the DM. Beyond the obvious reading of the module,in this case, the DM must put at work his fantasy. This is good but, for many, not enough.
Instead, looking at a fine map with the torchlights drawn on the marble surfaces of the walls, and the iron bounded wooden doors that open into the room projecting a beam of light into the stone floor, could suggest more things and helps the DM to add flavor to his descriptions.
Of course, better if you can use the map for miniature gaming.
That said, we produce the map using a combination of artistic techniques and 3d modeling. I design the maps using a CAD program (running under Mac OSX, if you are curious; in a past life I was architect), then my staff takes the map and makes three dimensional drawings, adding details (furnitures, lightning and more); then the work goes to the texture map section.
Here are built the textures representing the floors, the walls, the wood etc. At the end of the process a draft rendering is made and I correct details. Then we make a final rendering and the image is imported in Photoshop, where other details are added (like borders and drawings that cannot be made with a 3d program). Finally all the work goes to Illustrator where are added the writings, room numbers, key box, symbols and the like.
As you can see, it is a quite long (and expensive) process but this is part of our "quality policy".