When they're not teasing us with foldable phones that cost more than some used cars, phone companies have been spending their time at MWC showing off 5G variants of their phones. Samsung, Huawei, LG, Xiaomi, and others have been at it. Even if they didn't have a phone to announce, they've been promising that they're on the way. But Motorola is thinking differently. It's just developed a 5G mod you can attach to the back of the Moto Z3.
Modular phones never took off, and after the demise of Google's Project Ara people generally lost interest. LG tried it for a bit, and Motorola have been pumping out phones you attach a bunch of modular accessories to. Naturally, with the prospect of 5G inching ever closer the Lenovo-owned company has decided it's a good idea to give people the option to upgrade to 5G without having to purchase a brand new phone.
Why? Well the obvious reason I can think of is so that people can pick up 5G if they want to, without all that expense. Motorola has also referred to the 5G mod as a "whole Snapdragon smartphone" and has much better battery life than the competition. Of course there's no way to verify that claim, but if there's a full smartphone computing system inside then it makes sense that the mod would also have some battery cells inside as well.
As you can see, it's about as thick as the Z3 itself, and all that hardware needs to get its energy from somewhere.
The downside here is that we don't know anything availability. You can get the Z3 here for a pretty reasonable price, and with 5G networks coming before the end of the year it makes sense that Motorola would offer the British a chance to upgrade their handsets. Unfortunately all the information on the stand said was that it was exclusive to phones sold by US network Verizon, though compatibility with older devices may happen in the near future. Honestly, we'll just have to wait and see, though the way things are going it might end up being another EE exclusive.