Why do we have dreams? Do you believe behind every dream there is a hidden message?

I was just wondering what people thought of dreams I personally think they’re how our subconsious tells us what we desire. As for the hidden reason part. I think it depends on the dream it’s self. Weather you dream about the past and wake up and realizing something you hadn’t before. Or if you have a dream and don’t think anything of it then one day what you saw happens like day shaw vu (I know that’s not how you spell it but I can’t remember the actual speling sorry) So what do you think of dreams? Thanks.