Smaller and lighter form factor for those people who don't like big phones.
"The Moto G4 Play measures 144.4 x 72mm with a curve from 8.95 to 9.9mm and it hits the scales at 137g, making it slimmer and lighter than the third generation, as well as the other fourth-gen models. The measurements for the Moto G4 Plus and Moto G4 are 153 x 76.6mm with a curve from 7.9 to 9.8mm. They also both weigh 155g."
Well except that there's a latent pre-existing distribution system already in place for electricity. To the author's point, it's going to be converted back into electricity for locomotion, so at that point it's basically another form of battery, but with no existing infrastructure for delivery at this point.