to dry out your clothes.
unless you have a shelter, staying dry can be difficult as long as it is still raining. a plastic bag poncho will still keep the worst of the rain off you, but it can also block off the movement of sweat away from your skin. as a result, you must be very careful to pay attention to when your clothes feel damp, or open the bag up to allow it to vent from time to time. needless to say, you will not be able to use the bag as a covering when sitting by the fire.
unless you are camping during a complete social collapse, you might 