I don't think there is objective scientific evidence for dreams having a reason per se but there are psychological theories around mostly based on the work of Sigmund Freud, who used dreams to try and untangle the complex inner lives of his clients. Freud more than anyone (other than Jung perhaps) brought dreams into the realm of the every day lives of modern people. He dealt with hysteria, wish fulfillment, repression and trauma, creating the idea of the unconscious,dreams being the road to that strange world. He used symbolism to help his patients but never worked alone with dreams, always using a person's circumstances to help unravel problems.
Most other theories work on the idea that all characters and objects in a dream represent aspects of the self - your personality revealed in disguise.
Perhaps Freud is worth looking at in your case because he comes closest to giving dreams a real function in the psychology of modern humans. I don't know if dream therapy 'works' having never tried it but I did have a deep interest in dreams some years ago, using images for art work and story ideas.
Most psychologists think dreams are a shedding of material we repress in our conscious life, manifesting in images and surreal events as we relax and our brain waves alter during sleep.
Dream interpretation is an art and I don't think you'll get any joy whatsoever from the popular online dictionaries.Best to find an intuitive listener with an interest in Freud, or a psychology degree!
As for predictive dreams - well, anything's possible I suppose but if they were a shared and known experience, common to all, the world would be hearing about them for sure. I have never had them so have to go on my own experiences and say that I think they are unlikely.
Dreams are wonderful. I sometimes fly in mine and wake up so fresh and full of joy I want to fly in real life!! Great! I love the way dreams affect emotions but have to say that. depending on which type of cheese I eat, dreams aresoften just random chunks of life made surreal by the brain.