What3Words is pretty cool to play with but Im not sure of its practical use, at least in he U.S., Europe, and other developed regions of the world. That said, I can visualize this as tremendous boon in less-developed countries.
For me, I can visualize using this system in geo-caching competitions, locating camps in the wilderness, and other outdoor activities.
For those inclined to more nefarious activities, you can set up semi-clandestine meeting locations.
One addition Id like to see (maybe its already there and I just havent found it) is full integration with Google Maps and other navigation apps.
I havent spent enough time with the app to give it 5-stars but it adding that integration, especially for the third-world countries, would take it there.
Brass Tack about what3words: Navigation & Maps, v2.8.3