Ummm... so **someone** has to have a smart phone and an 'app' in this process?

No it just has to be known.

In Nigeria and India - if you are affluent or needy enough to have something delivered to your house - you will have found out your lexicographical coordinates. Your local post office will look it up for you.

If you are a hill walker and you slip and injure yourself, you will wish you had taken a SatNav or smart phone with an app. This is one method, and it is easy to say three words over and over, and those three words, presumably are from a set chosen to not be easy to be misread for similar words. Saying a load of numbers is slower and harder, especially if you are in pain. Plus is it LongLat, LatLong, OSGR letter-number, 4 digit, 6 digit. The receiver hopefully is recording/writing down. But in emergency things aren't always that orderly.

And after an earthquake?

what3words has its uses, and very good ones where there is no adequate alternative in place. Nigeria Postal service ie.
The UK already has an established system, it will be the one of choice but it is far from perfect.

for my OS Bench Marks in my locale website - I use OSGR letter/4+4 digit visible with a 6+6 digit in the URL. And have added what3words as alternative. Post code would not help with a canal bridge in a field with no visible canal channel (Lottery bid may change that.)