What is a Diesel Prime function and why do I need one?

If your engine is modern (post 2006) and has direct injection it is unlikely to have a heater system to start the engine.The engine control unit (ECU) takes over once you press the start button in your car and a few seconds latter the engine starts itself. 
Most boat diesel engines are vehicle derived but if its a 'home' conversion the ECU remains on the car/van.
These engines MAY need the diesel system pressurizing to purge the system of air/moisture before the engine turns over.
That is what the diesel prime function does on a Dragon panel. Once you turn the ignition on the diesel pump live is activated and the start function is not allowed to work for ten seconds. So your diesel is up to pressure and all air/moisture should be gone from the system before the engine turns over.