Google Maps is a great product, but people are still mostly bound by their 3G connections all over the world, and by their limited amount of data that they can use with their plan. This has always been one of the biggest downsides of Google Maps compared to other offline maps apps, that usually came from GPS service companies.
Google has released some features over the past few years that would make Maps use a lot less data, for example, with the transition to vector-based maps, instead of the image/tile ones we had before. When they launched it, Google said that the new Maps app can use up to 100x less data. It also made for a better visual experience, thanks to a smoother zoom in and zoom out, instead of seeing chunks of the maps showing nothing until those particular tiles were downloaded.
Then we've had the precursor to what Google is announcing now - a feature that could be activated from the Labs settings in the Maps app, and it would allow you do download a 10-square mile area, around the address you would select on the app. Now Google seems to have completely baked this future, and they would allow you download multiple areas, and probably bigger areas as well.
The feature is polished enough that it even works with a compass, even with the offline maps, so it should still be able to show you which way you are heading on the map, even when you change the direction yourself. It would've been even better if it worked with the GPS as well, so you can use local offline maps with GPS for Navigation, but it doesn't look like it's going to work like that, or at least Google hasn't made any mention of it. Google also hasn't given a specific date for the launch of this feature, but it seems that for now it will only be available for Android.