Dreams are part of our biology in that we're animals of perception first and animals of conception second. Our memory systems are so complex that often we can get lost in memories, dreams are just one example of this. Especially, when the day's worth of experiences trigger very old memories. Oddly enough, I can sometimes use dreams to do work on programs or my other work (theory of mind/ai), which is useful, or to even learn something new (going over what I've learned in a class or a book). There may be no inherent meaning to dreams, but this utility to them make them very useful and valuable in my opinion.