The arc creates Radio Frequency energy that is radiated. This energy is made up of many very high voltage spikes that can destroy modern solid state devices. A good ground can help prevent or reduce the amount of radiated energy.
Disconnecting sensitive electronic components such as ECMs is to prevent that type from EME (Electro Magnetic Energy)from damaging them.
The very first radio transmitters were simply arc generators that were turned on and off to send the dots and dashes of Morse Code. They were known as Spark Gap Transmitters. This is why radio operators were often called "Sparks". Spark Gap transmitters are now illegal since it is impossible to tune them accurately.
