Some trucks idle at rest stops to keep the engine and fuel system warm especially during cold weather.
It's also done to keep the diesel fuel from gelling up which would then cause the engine to not start up again.
Or if the truck is hauling a refrigerated trailer they are running the truck to keep the refrigeration system running in the trailer.
Some truck drivers also leave their engine idling to save wear and tear on the starter motor, to keep the air conditioning running to keep the truck cool or during extreme cold weather they want to keep the interior of the truck nice and warm and to prevent the truck from getting too cold and possibly have a problem restarting the diesel truck in the cold weather.
Diesel trucks can be very hard to start sometimes in really freezing cold weather.