Many cat owners prefer dry cat food over wet food simply because it is convenient. Dry food can stay out all day and not spoil and is generally less expensive than canned. But canned cat food is closer to raw food and contains a lot of water, which helps cats get the hydration they need. Canned food also generally contains less filler and carbohydrates, though reading the labels carefully is key.
For certain health issues, wet food is preferable. For cats with kidney or bowel problems, dry food can exacerbate the condition, and if your cat has urinary issues, the water content in canned food can be especially valuable.
Experts, including About.com's Franny Syufy, the ASPCA, and CatHealth.com and Pets/WebMD.com emphasize that wet food should be your cat's main food. Some, such as veterinarian Lisa A. Pierson at CatInfo.org, go even further, saying that dry food should be eliminated altogether. "I would much rather see a cat eat any canned food versus any dry food -- regardless of quality level of the canned food," she writes, going on to include supermarket-grade foods in that assessment.
Still, most experts say that small amounts of dry food can be used for variety, convenience and cost savings. Feeding half-dry, half-wet is still better than feeding only dry. Small amounts of dry food may be a slight help in keeping cats' teeth clean, though some experts say this is a myth.
Sources: About.com, ASPCA.com, CatHealth.com, Pets/WedMD.com