[robotresearcher]

If your head is down in the car, you lose track of what's going on outside and have to reaquire situational awareness from scratch when you 'wake up'. That takes longer at best, and you may miss something important at worst.

Small things like bikes and pedestrians are frequently occluded and you may miss the glimpses of them you needed to predict their future trajectory.

We could argue about the absolute importance of this effect, but it's definitely real.

[bobbylarrybobby]

Especially when you don’t notice the light has changed, someone honks at you, and now you have half a second to reacquaint yourself with what’s going on in front of you before stepping on the gas. I see many drivers looking down (presumably at their phone) at a red light who get honked at and almost immediately take off without any safety checks. If, say, a child had run out in front of the car, that child would get run over.