For one, a lot more people have powerful mobile devices than they have powerful desktops/laptops. Second, price-wise, you can sell the VR device a lot cheaper

For one, a lot more people have powerful mobile devices than they have powerful desktops/laptops. Second, price-wise, you can sell the VR device a lot cheaper if you rely on the user's existing phone to power it, making it more affordable. So it looks that it's going to be the other-way around: get a more basic mobile version first that will finance the more more tech-savvy (and more demanding) desktop audience.