Pobably because some of those dream's books are full of potential ideas, either for him to use to conquer the whole multiverse, or for peoples to stop him. Barbatos was known by some peoples for thousands of years. Some have to have dreamt to write book about him, books telling how to push him away, perhaps even destroy him. If Batman, Superman and possibly Cyborg manage to reach out to Dream and learn of all those things, Barbatos may be in danger.