In the Export Persona, when exporting an image for the web and you want the size as small as possible, you want to see a preview of what you will be exporting. So you can change the quality of a jpeg and see instantly what the result will be. The same when you change the number of colors in the palette of gifs and pngs.
Also show the sizes in bytes of the slices you are exporting. The size is shown in the simple Export, but not in the more advanced Export Persona.
Fireworks does this very well, it even has multiple previews with different export settings.