1. While the layers method would work perfectly well in photoshop, in my map each mountain, tree, and hill is an individual (and editable) object. I can move, resize, flip, etc. every single object.
2. The vector map is scalable. I can reduce or enlarge portions, crop out sections to use as the basis for more detail, and alter line widths and the like to maintain the aesthetics. In photoshop, you can certainly resample the image and crop it, but you will be stuck with the line widths when you zoom in.
3. My original map in the computer is 24x30". I had it printed on canvas and it hangs in a frame on the wall of my game room. I designed at that scale for that reason. I have a hi-res jpeg at that size stored on the computer, but it runs somewhere in the neighborhood of 12Mb as a flattened image. I would hate to use that file as my working one with the ten or twelve layers I have in the CorelDraw file!