Why do people hang clear plastic bags over doors of stores and houses?

I see those all over especially in restraunts where you can eat outside or in a porch. What is the reason for this?Why do people hang clear plastic bags over doors of stores and houses?
It prevents flies from going on the porch. It somehow it reflects the sun light and it scares the flies away.Why do people hang clear plastic bags over doors of stores and houses?
its to keep flies away.... some even have pennies in it, but i really don't know how it keeps them away... BUT IT DOES!
I think its supposed to keep the flies away
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