Basically, you can download any map of anywhere in the world and refer to it, or use it for navigation. This means your phone is effectively your sat nav or map. The maps are incredibly detailed, and this app, normally costing £2.99 is free for a limited time.
You dont need a data connection or wifi to view and use the maps. They are lodged on your device.
Those who travel from place to place will find it useful. Others wont :)