Why do diesel trucks have a plug to plug into an outlet?

It is to keep the Engine Block warm, so when you need to start the engine, it is not so cold that it won't fire up. Diesels have no Spark plugs so the the fuel is only Ignited by the high compression of the Diesel engine. If the block gets too, too cold, the fuel will never get hot enough to fire, ever with the glow plugs.

It is for cold weather. It is a block heater. Below 32 you plug it in and it makes starting the truck easier with less wear and tear.

That's the auxillary plug for models in Europe to be able to plug into the Audobon. That's why they can drive so fast.