Comment by andrewaylett

There are two layers, both relating to concentration.

Driving a car takes effort. ADAS features (or even just plain regular "driving systems") can reduce the cognitive load, which makes for safer driving. As much as I enjoy driving with a manual transmission, an automatic is less tiring for long journeys. Not having to occupy my mind with gear changes frees me up to pay more attention to my surroundings. Adaptive cruise control further reduces cognitive load.

The danger comes when assistance starts to replace attention. Tesla's "full self-driving" falls into this category, where the car doesn't need continuous inputs but the driver is still de jure in charge of the vehicle. Humans just aren't capable of concentrating on monitoring for an extended period.