1. Raw framerate - there's a reason that the Rift's recommended hardware is a GTX 970 with an i5 due to minimum framerates needing to stay above 90 while most consoles run at 30.
2. Head tracking - Cardboard relies purely on the phones accelerometer which, while good, isn't perfect. Having an external camera allows for far more accurate tracking along with no drift.
I own an oculus rift (a DK2) and can attest that even my PC with 2 970s in SLI and an i7-5930k struggles on some games to maintain the 90 FPS and it definitely makes me queasy.
Biting the hand that feeds IT
