A lot of designers start off by putting web layouts together in programs such as Illustrator or Photoshop, to quickly give their clients an idea of how it will look. When they begin to work on the real web page, they might use slices to isolate the text, for example, and then export the whole thing to a HTML file. This is where Dreamweaver takes over, importing the complete HTML file, and making the necessary alterations. That's just one use of slices, I'm sure there are many more!