The official Google Maps blog:
Roughly 60 percent of the world is without Internet today, and even where online access is available, it can still be spotty. That means that quick and easy access to information is still not possible for a majority of the population. This is a huge problem, especially as people attempt to navigate and explore the world around them, so Google Maps is taking steps to help people across the globe find directions and get where they’re going, even when they don’t have an Internet connection.
Now you can download an area of the world to your phone, and the next time you find there’s no connectivity—whether it’s a country road or an underground parking garage—Google Maps will continue to work seamlessly. Whereas before you could simply view an area of the map offline, now you can get turn-by-turn driving directions, search for specific destinations, and find useful information about places, like hours of operation, contact information or ratings.
This is a huge benefit, especially for folks with limited data plans. By far, the biggest suck on my data plan is map usage when I am on the road. Love this!