Sigmund Freud: Dreams are a secret outlet for repressed desires.
Carl Jung: Dreams are a way for us to compensate parts of our personality that aren't properly developed in real life.
OneirologyTheory1: Dreams are a way to help us cope/deal with emotions of everyday life and solve problems that we are currently experiencing in our lives.
OneirologyTheory2: Dreams are a way for us to organize information that we have learned through out our day.