A Google blog post explains: "Detailed floor plans automatically appear when you’re viewing the map and zoomed in on a building where indoor map data is available. The familiar 'blue dot' icon indicates your location within several meters, and when you move up or down a level in a building with multiple floors, the interface will automatically update to display which floor you’re on."
Some of the the places mapped make perfect sense. Airport maps, for instance, sound very useful. Sometimes, when you're sprinting to get on a connecting flight, you just don't have the time to grab an employee and ask for directions to a terminal that's a half-mile away. Other mapped locations on the list seem pretty silly. Macy's? You really can't get yourself from shoes to neckties without a map? And doesn't Home Depot make a big point to tell you in ads how helpful its staff members are? Surely somebody can tell you where drills are in relation to lumber.
One thing new inside Google Maps can't help you when you're waiting out a delay at KCI: finding decent food nearby. There isn't any! Ooh, burn.