Ever seen one of these?

Ever seen one of these before? I only have a few times. It’s a Ziploc baggie half filled with water hanging from a door jam. I have seen it at a Mexican restaurant in Seattle, a candy store in Lincoln CIty, OR (as seen here), and somewhere else as well. Supposedly it’s supposed to keep bugs out of restaurants who leave their doors open during the summer, but that does not seem logical. I have heard some people say that because insects like flies have hundreds of eyes- that the hanging baggie of water appears to them like a wall of water blocking the entrance, but that is ridiculous. I couldn not find any logical information online. What do you think it’s for. Leave a comment below.