This is because the brain needs time to process the information, form sensible pattern out of it, and place it in long term memory. Dreams can also improve your emotional well-being, reduce stress, improve your creativity, and provide a playground for your mind while your body recovers and repairs itself. Actually, this isn't true. Supposedly the scientist in question had a dream of a snake biting its tail. Anyway, Watson and Crick 'discovered' the structure of DNA using models, not dreams.
Many people gain creative impulses from dreams.
Many different theories, nothing for sure. According to the Freudian school, dreams are the result of subconscious thoughts and desires. My own understanding is that dreams are made out of many small parts of memory and imagination that get combined to form dream imagery. I don't know and leave to speculation the reason why this is so. Often, the subjective time spent in a dream is much longer.