Photoshop :: Exporting Design Into Dreamweaver - Why To Slice Image

I'm exporting a Photoshop design into Dreamweaver. I know HOW to slice, but I'm having a hard time finding the reason WHY to slice an image for importing into Dreamweaver. Why not just import the whole image, as a single, whole image?
 
I'm also having trouble finding the best way to slice an image. This is all very new to me. I looked at a webpage I created almost 5 years ago (my first exposure to web design) and I sliced the top of the page into 3 images - I had two prominent images on either side and the middle was the background. I then divided the middle bar (with the buttons) and made each of the buttons their own slice/image.
 
However, I just came across a website where they ultimately have their design sliced into just three images - a logo, buttons and a top layer image.This is the site I came across: URL.....
 
My slicing looks something like the opening image under the "Killing the Clutter" heading of this website, not the closing image (their goal) under the same heading.
 
Why are they singling out the logo? Should I be doing this with my design?Why are they including all buttons within one slice, instead of individual slices? Is this OK to do? For purposes of the rollover image, I thought they had to be separated.They're not saving the background. Should they be? Unless they're importing the background as one large image.
 
who might recommend not slicing an image and doing stuff with code instead, but I don't have the knowledge to go that route and I have to slice the image.