As the winter arrives here in the UK, we expect a lot of rain. The horses which live out - even for only part of the day - are mostly rugged up especially if they have been clipped out. If they get too cold, then they lose condition.
My horse at the moment wears two coats - the top one is waterproof and is fitted with a hood. Some owners use detachable hoods - for use only when the weather is very bad.
The rug keeps the horse warm and importantly clean. I always fit a rug in the winter months. Looking in the tack room, she has 8 rugs of different weights for use throughout the year.