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.
* * * * * * * * * * * *
First hermit: Still there’s one thing about being a hermit, at least you get to meet people.
Second hermit: Oh yes! I wouldn’t go back to public relations.
— Monty Python, The Hermits
(I shall be offline from 27th June until November)