These are functional instruments that quickly indicate any changes in atmospheric pressure. They are interesting as art, but they work! The air trapped by the liquid maintains a steady pressure. When high pressure (good weather), the liquid is pushed down the spout. When low pressure and bad weather invade your region greater pressure trapped inside causes the liquid in the spout to rise. If liquid spills out of the top you should take shelter!
I love science and art! There are so many beautiful scientific tools out there, like these styled barometers. I like the fact that I don't have to turn to my phone, computer, TV or radio to get an accurate weather assessment.