This mostly helps us animators though. As you can see at the signature below (dbz sig), it has an animated part (right side) and a non-animated part (left side). If I would save it as a whole. The image size would be much bigger, as .gif hate transparent images and there is a much bigger part animated (what actually is just static). So I sliced the signature. The non-animated part is saved as PNG-8 and the other one just as a .gif
Oh and you can also use it when you want to optimize your transparent signatures. For example:
(Imagine the tea part non-animated)
I could slice this image in two parts, lower part and upper part. The lower part would be JPEG (as jpeg has better image quality than PNG). The upper part would PNG, simply because JPEG doesn't support transparency.