Basically, us use this API when yo want to create some DHTML that will work across a wide variety of browsers. This includes hierarchical menus, tabs, and other useful navigational and information-serving stuff.
Of course, I suspect some of you might be tempted to use this for evil instead of good … such as cursor trailers and pop-ups … DON’T DO THAT! Gizmos == Junk! Junke == No Visitors.