Since we are using the slicing tool in Photoshop for our web pages, here is a little article I found...

In addition to optimizing, other reasons for slicing Web graphics are:
1. when you can repeat or stretch a single small image to fill a larger space,
2. creating mouse rollovers and clickable image maps
3. animating portions of a large image.

Here is the link to the rest of the article... http://graphicssoft.about.com/od/webgraphics/a/whyslice.htm