its to idle down your turbo, even the owners manual suggest you idle the engine for 1 minute before shutdown,
DO NOT REV THE ENGINE AND SHUT DOWN, as I see all our diesel using population do. It starves the turbo of oil supply as it idles down from 20,000 rpm or so. Ive found that running the engine at 1000 rpm for the last km of your drive is best to idle down the turbo with a steady oil supply, once you reach your destination - shut it off.