What on Earth is what3words' business model? Did they expect to own the concept of using words as coordinates?
Once they've achieved this (either informally or by getting enshrined in the legal system) they can start squeezing local businesses and the government for access to their service. It's pure rent seeking.
As part of this strategy, it's in their interest to create an unnecessarily unintuitive service so you can't go anywhere without using their app. The three words provide absolutely zero geographic information about the location.
They're about as close to a truly evil company as I can think of. Luckily they seem to have had minimal success so far, as the revenue numbers are awful.
Addresses typically correspond to a single building or subdivided unit. Your home/apartment/factory could overlap several GPS grid squares, and then people have to decide which one is canonical or record all of them.
In most places, addresses are roughly sequential. This gives you the ability to reason relatively: If I just walked past stores numbered 2503 and 2505, and I'm looking for 2751, how far away am I and am I going in the right direction? Easy to estimate. If I'm looking for manatee.fracas.bologna and I'm at porkpie.hamster.obnoxious, I can't make a conclusion.
Sequential addresses also make sorting and routing easier. Group the items by street name, then sort by number. No technology required, and easy to monitor and fix if it gets upset. Shuffle a few envelopes in a W3W mailbag, and it's going to be a lot harder to get it back in order.
They could also probably sell novelty "phone numbers", like every professional sports team buys the equivalent of "boston red sox" for the location of their stadium.
No clue what their actual business model is, I'm just spitballing.