@freemo

They seal up a loose collar that might let the wind inside your clothes. The rate of heat transfer from your skin into air scales with the temperature gradient, which is reduced by keeping cold air at a distance.

I also wear one while motorcycling in the winter because exposed skin gets very cold at speed (high speed = thin boundary layer = steep temperature gradient). As a bonus, the scarf tucks over my nose and beneath the pads of my glasses to direct my breath away from the visor.