Rain and snow can get in the trailer connection to the car and blow the fuse disabling the trailer lights. I know there must be some high-priced cover that you can buy to alleviate the problem but the plastic bag has been working fine for me.
I keep the bag on the trailer connection all the time while it’s parked outside in rain and snow. When it comes time to hook up the trailer, the connection is clean and dry.
Cost: Pennies. A lot less than replacing fuses every trip.
Photo by Pete Zimowsky/Idaho Statesman