The lighting creates warm showcases and thus are not good for the products place in them like gemstones, electronic stuff and even photographic gear. Humidity levels can create some surface to lost lustre or tarnish...etc. The small glass of water placed inside the display can in a way balance the dryness caused by the lights.
Another reason can also be due to feng shui and that usually would mean it being place at specific locations of the showcase interior or even exterior.
So I guess you can combine both to maximise both durability and money at the same time heheheh