Water is essential for every living animal and cats are no exception. Even though their ancestors came from a desert environment and relied on getting adequate amounts of water in the food they ate, modern day cats eat lots of dry food. This dry food, which has very low water content (about 10%), means that they need an additional water supply in the form of a separate water bowl.
What most people don’t know is the fact that some cats don’t like having their water close to their food and, therefore drink less (or won’t drink at all) if you have them next to each other. The reason behind this is that they feel that the water is contaminated.
So always keep the containers separately so your cat is not dehydrated.