Traditionally this spot is where the most news worthy piece of information is placed in the newspaper and often has a picture that accompanies it. On a web page the position retains its value as being the first thing that readers come into contact with.
Advertisers want their ads placed above the fold because it is proven that there is significantly higher views on ads which appear on the screen without the need to scroll for them. Site options and the main features will always appear above the fold in case the page visitor does not scroll down. This would include marketing the best offers and having links to the main pages. The parameters of size is changeable depending on the type of screen the site is being viewed on so the main information will be highly concise to fit the smallest screens.