Soaking chopped hay makes it easier for horses to chew, and can also reduce the chances of your horse choking on chopped hay. Damping down chopped hay will make it less dusty and also provides your horse with extra hydration.
If you choose to soak chopped hay, the water should be added shortly before the hay is fed to your horse. Soaking chopped hay further in advance can lead to the development of potentially toxic molds and spores.