when you have a site, do you have (for example a banner) a table with separate images inside the table to make a banner?
If you mean say split the banner image into 5 images, then display those images next to each other in a hidden table to show the complete image you could do that. may help load time a bit. but if the banner is small(jpg) then i dont see the use for that.
I think the main reason people do that is so that Javascript or some sort of dynamic thing can have a mousover affect over one part of the image without changing the whole thing. Like websites you go to with text on the picture, and when you put your mouse over the text the text will change color but the picture will stay the same. That is most likely javascript and the hidden tables.
its probably for both reasons, load time and for possibility of mouseovers if your doing table layouts then splitting the images just might be the easier way to slice your design...
It also depends how you are designing your pages. If you design your pages completely in Photoshop, you may want to slice them up into tables and then display them as separate images. This definitely helps load time. It depends on if you are a visual designer or more of a coder adn how much CSS you want to use. I used to use a lot of slices in my sites, but it is good to switch it up.