Why do our brains dream?

I want to point to a review by Nick Romeo of Zadra and Stickgold's new book "When Brains Dream". The review summarizes important theories about dreams and gives the authors' own model:

...Though they tour a broad range of contemporary research and theorizing, they ultimately propose that a primary function of dreaming is to detect and dramatize the possible meanings of information latent in memories and associations that we rarely access while awake...Their own theory proposes that dreaming extracts new information from memories by discovering and strengthening previously unexplored associations (they brand their model with the acronym NEXTUP: network exploration to understand possibilities). For this capacity to be a target of natural selection, however, the new information that dreaming discovers must provide at least some periodic survival benefit. They could be clearer in asserting this directly. They could also distinguish more precisely at points between the benefits of sleeping and the benefits of dreaming per se.