If high current capacity cable strands break as a result of flexing or vibration
and then come into contact with conductive material, significant sparking can
occur, which in turn can start a fire. To reduce the risk of this happening, main
and starter motor leads, which carry high currents, must have soldered or
swaged connectors which will resist flexing and protect the cable strands at
the terminals.

Spark plug leads must be supported so that they don't touch the engine
block and cylinder head, as their insulation could become damaged by heat.
This could cause them to spark and ignite nearby fuel. [3.6]