Dry dog and cat food should not be wet down if it is not going to be eaten in one sitting.
Moistened dry dog and cat food breeds bacteria. Part of the process that preserves the food for a longer shelf life is the dehydration.
Moistened dry dog or cat food should be left out and treated as if leaving out wet dog or cat food. It should be picked up and discarded, when you would normally consider wet food has been out too long to be healthy.
It doesn't need to be treated as if it is toxic after half an hour, by no means, it can sit there, but not for same the prolonged period that unmoistened dry food can be left out.
- Promptly refrigerate or discard unused,
leftover wet pet food and containers (e.g., cans, pouches).
- Refrigerating foods quickly prevents the growth of most harmful
bacteria. Refrigerators should be set at 40 degrees F. The accuracy of
the setting should be checked occasionally with a refrigerator
Makers of dry dog food also advise this:
- Dry foods remain fresh over many months because bacteria can't grow
and spoil food when moisture levels are low.
- Dry food packaging and
the fatty coating of biscuits help prevent the entry of moisture and
oxygen that can cause food spoilage during storage.
- Once opened, store
dry foods covered in a cool dry place. Do not store moistened dry
foods as moulds may grow, which can harm your pet. Purina
Another issue, which was discussed by @Baarn is that a dog (or cat) eating dry food will need to drink more water to counteract the dehydration processing of the food. The dry food is formulated with salt in a way to increase the pet's thirst, so they will crave more water, when eating dry food. If the dry food is wet down with water, this water contributes to the water needed to counterbalance the water needs of the pet. Any healthy pet will make up for this needed fluid b drinking.
It's always important to have an ample supply of fresh water for your dog or cat, especially when feeding with dry food, unless a qualified Veterinarian, who has examined your pet advises you not to do so.
Veterinary Science Animal Husbandry
American Veterinarian Medical Association PDF
Centers for Disease Control and Prevention USA