Why idle?

IN many parts of the country (US and Canada) they would ticket the driver who leaves their vehicle at idle unnecessarily.

I'm not an expert. My tractor mechanic was working on my swather. I asked him why the diesel engine ran so well. I drove it out to the field and it zipped right along at 6 or 7 miles an hour. Once I started cutting hay it just bogged down and had no power and the engine died. He said an idling diesel requires almost no fuel to keep it running. Once you apply the load it then starts to consume the fuel. Very little expense letting a diesel run Vs turning it off and back on and warming it back up again. The diesel probably takes more fuel starting it and warming it up than idling while making deliveries. If that delivery man shut that diesel down at every delivery all day long seems it would cause more problems. Very different from gas engines.

A swather is a 14 ft wide (in my case) lawn mower that cuts hay and puts it in swaths on the field so it can dry.

Can't speak for the operator of the delivery truck or the reasons why it was left idling for over an hour...but,

Mine gets shut down if I'm not driving it...(unless I'm letting the EGT's -exhaust gas temperature- come down after a long hard pull. This takes about 5 minutes or less)-I have a pre turbo gauge.

Also, an unattended vehicle left running is inviting car theft.
Shut it down, leave it and lock it.

Makes no sense in summer. In winter there are several good reasons, including operator comfort and the fact that older diesels were hard starting when cold.

Depending on what vehicle it was. I have heard on some of the newer ones that they have a regen cycle to clean the DPF filter in the exhaust that requires a higher idle for a period of time. Owner might have just left it to complete the cycle and forgot. Or it was a company truck and they don't pay for the fuel.