Hot and cold is run totally separate in the UK, I still don't think this would do anything...

I'm not sure exactly what you're saying here, but I'm assuming you mean they run each source in separate lines. That's the same in the US. The point is that these pipes aren't well insulated so the line has probably cooled even though it is a "hot" line. Running the water a little bit flushes the cooled water out so that only hot water goes into the dishwasher. Because even though it is a "hot" line it gets cold over time if it isn't being cycled. Maybe in the UK you're much closer to the hot water source, but in the US houses are often so big and poorly designed that you need to run the water a bit before you get really hot water from the tap.

At least in Germany, dishwashers are hooked up to the cold water line. No matter what I do with my sink, the dishwasher will never get hot water on its inlet.

Thanks, that was the context I was missing from the parent post. Makes total sense.