Out in the wild, way back when, our companion animals hunted for their food and ate what they caught. The moisture available from the animal food source was high, about 70% moisture, plus the animals drank water in addition to what they ate. Now think about what our pets eat most of the time now, dry kibble that contains about 10% moisture.
Not that kibble is terrible and of course some are better than others. That said, there is still little moisture in a cup of kibble and that's a problem.
One way to deal with this discrepancy is to provide moist foods in addition to the dry food. One can do this by simply adding some warm water into the dry food and letting the water absorb some of the flavor from the kibble. Freeze dried foods where you add water to rehydrate the food is another great option to add to your pet's diet. Canned wet food can provide additional moisture as well since this type of food is about 75% water. Of course read the labels and choose a wet food that contains wholesome ingredients!
Cats in particular are less inclined to drink from a bowl (non-moving water source) so it is recommended to provide a water source that appeals to your cats natural instinct. Even still, your pet is more than likely not drinking as much as he really needs so it's important to sneak some moisture in your pet's diet so he gets all that he needs. You pet's organs will thank you!!
Continuing the theme of moisture and diet here is a link to another really interesting article on moisture content in dry food and how to get a better reading on the actual amounts of protein and fat in your pet's food: http://www.dogfoodadvisor.com/choosing-dog-food/dry-matter-basis/