While Apple's tech used for Face ID on the iPhone is impressive, it's debatable whether it's more convenient than Touch ID. There are also concerns that your face could be used to track shopping patterns or be seen during mass surveillance by intelligence agencies. More importantly, it could be easier for law enforcement, and even thieves, to force you to unlock your iPhone.
In the US, law enforcement officials can make you unlock your smartphone with a fingerprint, but they can't force you to input a password or PIN, which would violate your Fifth Amendment rights. To help you from ever being in a scenario where you're forced to put your finger on the Touch ID sensor, Apple has a built-in way for you to disable biometrics on your locked or unlocked iPhone in mere seconds.