My daughter showed me this several years ago. I've used it on the inside of a bottle holder I made. The bottle of cold water sits down inside it, and the cold water stays cold longer, and it doesn't sweat out into the fabric, so the outside fabric stays dry.
We do it with anywhere from 8-10 bags together, not folded, and make big pieces that you can cut to fit things! My daughter even used a really big one she fused, to wrap a gift for a bridal shower.