Snakes are brumated to increase fertility and to provide an additional trigger (in addition to light cycling) to breed. Conventional wisdom has it that males will be more fertile if cooled. Females are cooled too, to keep both snakes on the same schedule.
Some corns live in south Florida and may never experience cold temperatures. These snakes would not need to be cooled. Other corns are native to New Jersey, Virginia and Kentucky - it snows there. So cooling replicates what would be natural if they were in the wild.
Most people do not know the locality history of their corns, so they cool them "just in case." Though there are a number of breeders that do not cool their snakes at all and still are very successful.
Many keepers simply like to take a break and cool their snakes so they won't have to maintain them in the winter - even if they're not breeding them.
Tim