Double manned vehicle and daily rest

madmackem:
For the first hour of multi-manning, the presence of another driver is optional, but for the remaining time is compulsory.

Admittedly I don’t double man but I’ve never really understood the logic behind this. If the second driver wasn’t picked up until four hours into the first driver’s shift, then so long as the rest period is determined by the first driver’s start time, then why would it matter?