The diode requirement is dependant on your wiring, and I don't have a schematic with me now. The purpose is to provide a better power connection (unfused and thick wire, so less resistance) to the electronic ignition while cranking the engine. Electronics don't like it when power dips , so grabbing power from the thick starter cable while cranking is a good idea especially when the battery is getting old and can't keep it up any more. There was also a problem on the late 140 and the 240 series whereby the fuse box can get wet from a leak by the windscreen. It was a bad place to put a fuse box!

But running without a fuse is also a bad idea, so after the engine has fired up and the starter motor stops you get power from the fuse box. And the diode prevents sucking power out of the fuse box to the starter motor which would start a fire ;-)