Make up for a lack of logical document structure, content of value, or accessible design concepts... in fact in a lot of cases it actively encourages the exact opposite of all three.
Though it can also be used when you have all three to force a layout the markup structure doesn't match -- assuming you can get Safari to play ball with it aging like milk so badly it's becoming the new IE6... what with the Apple rotting on the vine thanks to Google absconding with all the talent in forking off "Blink" from webkit.