This post is meant to inform you of the reasons to feed some canned food (not ONLY dry, in other words) and some issues that can come up if you do not. Firstly, let's talk about the way cats are used to getting the majority of their water. Cats do not have a very strong "thirst drive" so do not drink a whole lot of water from a dish (some cats can be encouraged to drink more if the water is moving, like it is in a fountain waterer) normally. This is because they normally would get a lot of water from their food (a mouse is about 70% water). This lack of a thirst drive can be a problem because many cats eat primarily dry cat food (it is convenient, after all and many cats love it!). Cats that primarily eat dry food live in a state of chronic dehydration (sometimes mild, sometimes more serious depending on your particular cat's thirst drive).
As in humans, dehydration can (and does) cause "issues" with the urinary tract. Because of this, cats that eat dry food are more likely to have cycstitis (inflammation), crystals (stuvite, calcium oxilate, etc.), or urinary tract infections. By feeding even just a little canned food, you are making it much less likely for your cat to experience the pain and suffering of having a UTI. Think about it this way: each amount of canned food you feed "washes" out the bladder, making it harder for bad things (bacteria, crystals) to develop or stay in the uninary tract. There have been studies that showed that cats that do not eat canned food consume, on average, only 50% of the water that cats that eat canned food do. That is HUGE! So, while canned food may not smell good and may be somewhat less convenient and more expensive, it is something that should be viewed as a preventative and should be given to all cats.