It's for the venturi effect and allows the exhaust gases to be cooled before the. exit the tip, which then reduces the chance of burning someone or catching something on fire.

The 1,000 degrees fahrenheit exhaust during re-gen is pretty hot, with paper burning at around 480 degrees, or so we are told. I could understand why you might want to cool down. Diesel emissions technology really has helped to clean up the cities, even if it some places it's pretty silly like on farms and remote areas. But I guess you can't have one farm tractor to work the land in Southern California where pollution is a problem while in South Dakota there is little issues with air pollution.