Hard to say without seeing the detail, however without a relaxation you could easily get the situation for example where driver Bloggs goes out to Mrs Miggins farm in the borders to deliver fuel (and her neighbours), instead of his run taking the usual 7 hours or so it hits 9 hours driving and he's still on his way back due to the conditions, not much later he runs out of daily hours...
He may still feel fine to drive but not legally be able to, he should now pull in and take his mandatory daily rest, but where?? The verge is piled with snow, as are the layby's, a Police officer could maybe authorise him to carry on until he reaches a safe stopping place, but there are none about... so he's left to break the law (endangering his job) or block the road. Or call base if he has that facility to ask another vehicle to risk the conditions and bring *another* driver out to bring the truck back, assuming another driver is available...
To avoid this the company must double man the trucks, halving the number of deliveries they can do.
Or just not deliver to the difficult locations..
I'm all for drivers hours, imo they should apply to ALL business vehicles/drivers, but in these (exceptional) conditions imho it makes sense to allow a little slack.