You don’t want sewer gases entering the home. The water in the toilet will eventually evaporate and the film can help delay that as well as be a physical barrier to any gases that seep through after the trap is empty. It’s not perfect but it’s cheap. I’d imagine these have been drained completely though as it’s winter and the heat is likely off due to being a foreclosure. More evidence of this as one of the pictures shows snow inside the home.