Classic example is emergency services.
Having the ability to turn your location into three words that will easily survive communication over a phone with dodge signal is very useful.
You could argue that that lat/lon or a street address should be fine. But memorising a two sequences of numbers, then repeating them accurately over a poor phone line is hard.
Street addresses are also hard, there’s frequently not a nearby road sign, or worse, the road name is ambiguous (your city may have multiple streets with the same name). Hell there’s no guarantee that the emergency services even have your street name in their database if it’s a new street, or it’s name was changed recently.
So basically any scenario where you need to transmit a location accurately between two parties who are operating two different mapping systems, and communicating over an unreliable comms link (phone, radio etc), is a good use case for W3W. W3W is like the phonetic alphabet for locations.
Personally I hate the fact the W3W are extracting rents from this services. But I can’t deny the utility of their service.