It's called a saṅghāṭi and was originally prescribed for use in cold weather. It still serves this purpose for thudong monks and monks living in cooler climes, but for everyone else it is now merely decorative.
The Great Sage did thus proclaim
With many a diverse chosen name
Vipassanā stilled and purified,
That to Emergence is allied.
Saṃsāra-cycle’s swamp of pain
Is vast and terrible; a man
Wisely should strive as best he can,
If he would this Emergence gain.