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]