I love it when I open a new book and find a map on the first page. To me, a map is a promise made by the author to the reader: here is the world you will enter, and these are the places you'll go. Reading through a book is made all the sweeter when you flip back to the front and realize you only saw a fraction of what that world contains.
So here's a map. Incomplete, but workable. I have some loosely defined climates colored in, but geological features, political borders, and cities and settlements are still missing. At this stage when looking at the map, I have to ask myself what came first, the map or the story?
There are plenty of stories, written and unwritten, that I can populate this world with (re: the many Birds-Eye posts). Likewise, there are numerous nooks and crannies on this strange version of Earth that are ripe with potential happenings that are simply waiting to be filled in.
That's the nice part about a map. As I compile more and more snippets of a growing world, it's good to put it all together and see how the relate in a spacial sense. Not only that, but as the network of intertwining narratives of people, countries, and the land itself grows ever more complicated, it's convenient to be able to point to a spot on a map and say "That is where This takes place."
I am by no means an expert on anything, but consider that my recommendation to writers that like to build their world as much as they like telling its' stories. Make a map, make a promise!