Strictly physical protection for the cable. If the metal braid on your plug lead touches the sharp edge of a cowl or similar object, it can cut well into the cable and ruin it in less than a full flight! Protect it at all cost or you will have the pleasure of replacing the ignition before its time.

I prefer the split lomb type of cable duct over the spiral wrap style myself but that is just personal preference. If the lead can come in direct contact with a hot exhaust pipe, I add an additional layer of high temp silicone tubing so the duct can't melt. It doesn't matter what you use for protection, just use something you are comfortable with and keep the plug lead well away from anything that can abrade it.