The spiral will aid wind load shedding and hence flexing of the main element. When you drive along, the aerial gets a high pressure on its front and low pressure on its rear, because the main aerial is round this is not a stable state and the pressure areas move from side to side, imparting a sideways force on the aerial, causing it to oscillate sideways. The external spiral causes the air flow to be directed and hence stable.
The wind load shedding phenomena caused a 1000 feet tall Emley Moor TV mast to fall down in 1968. The mast went into oscillations.
Frank