Horses produce saliva when chewing with assists in digestion. When feed is wet, this reduces the production of saliva. Saliva contains the enzyme amylase which assists in the break down of carbohydrates. So whether to wet horse feed is a decision that needs to be taken into factor on an individual horse basis, linked with what feed the horse is being fed.
It is important to follow the manufacturer's directions when feeding. Some of these products advise that it is ok to feed without prior soaking, some advise that the product must be soaked. Do not take the advice to not soak a product that has directions stately that is must be soaked. It may put your horse at risk.
There's a few different reasons for needing to wet hard feed.
The feed itself absorbs excessive amounts of water
The horse is prone to choke
The horse has dental issues making it difficult to eat
1. The feed itself absorbs excessive amounts of water
Feeds such as copra, many soy derivatives and beet pulp all absorb a considerable amount of water - approximately 5 times the volume water. This means that whatever the horse ingests, water will be drawn from the stomach to be absorbed by the feed. It can also mean a horse is able to eat a larger amount of that feed if it is not wet, as it is a smaller volume.
For many horses this will not be an issue, as they will drink more to make up for this water absorption. For some horses with health issues, prone to colic, or older horses, it is better to pre-wet the feed, so that the horse is not at risk of colic from having a large mass of feed blow up inside the stomach.
2. The horse is prone to choke
Choke is when food becomes lodged in the horse's oesophagus. Horses are unable to vomit and so it can be stressful and even life threatening for a horse that has feed lodged in the oesophagus that they cannot swallow.
Wetting feed assists in making the feed less likely to form an immovable ball within the throat. The wet feed will more easily break up and be swallowed. A large mouthful of dusty chaff and pellets can form a ball that is difficult for the horse to dislodge.
The wet food is also easier to chew and swallow. A mouthful of feed that is difficult to chew may be swallowed before being chewed enough when a horse is hungry enough.
Please note, this is not the only precaution to take to avoid choke, this only in reference to wetting feed.
3. The horse has dental issues making it difficult to eat
More so for older horses, but for horses with any dental issues, wet food is easy to chew and swallow. Obviously by removing the crunch from the feed, the chewing puts less strain on the teeth and muscles of the jaw.
For instance a horse missing a molar will have loose teeth around the missing space. This makes chewing more challenging.