There are still downfalls when it comes to using your phone for navigation. The world is not blanketed with one giant internet connection. Some parts of the world have no access to an internet connection whatsoever. Also, navigating with a phone is heavy on data and battery life. At Google I/O 2015, an update to Maps debuted that combats those downfalls.
Google Maps will support location cards for recent searches and voice-guided turn-by-turn navigation even when an internet connection is not present or a device is in airplane mode. Google wants people in developing countries to use the features in order to learn about their environment in an efficient way.
The new offline features are expected to arrive later this year.