Thank you. Just habit. From what I understand they use a grid heater (also known as an intake air heater). The grid heater is an electric element located in the intake manifold that heats the incoming air to aid in cold starts. By warming the air before it enters the combustion chamber, the grid heater helps to ensure smoother starting in cold conditions. This system is different from glow plugs, which heat the fuel directly in other types of diesel engines.