Desktop areas allow you to keep a well organized system when you have many tasks to perform at one time. Just like adding a new desk when you have too much for one, desktop areas allow you to move to another area to launch more programs.
GNOME is aware of desktop areas even though they are controlled by another software program called the 'window manager'. You can set the number of desktop areas within the configuration of the window manager you are using. A fully GNOME aware window manager will have a tool within the GNOME Control Panel so check there first if you do not know how to configure your window manager.
IMPORTANT: Most window managers will give you the option of having multiple desktops which are different from desktop areas. Desktop areas are virtual extensions of one desktop whereas multiple desktops are actually separate.
The default setup of GNOME is to use desktop areas with only one desktop. The reason for this is because in older applications, such as those which use Motif, users can experience problems with some drag and drop functionality across desktops.