I agree with Crowv, I like dream sequences, but I think they should be used at critical moments of the story, otherwise too many could make things confusing (unless that's what you're going for).
Dream sequences are like time travel, for instance I wrote a character who imagines how things will turn out without them actually happening. It accomplishes two things. First, it gives you a glimpse of a possible outcome of your story while still maintaining a degree of open-endedness. Second, as Crowv mentioned, since it's from a character's perspective it allows you to show a lot about their thoughts and emotions without having to tell it in long expositions.
I know traditionally dreams can have deep symbolism as well, so there is that layer you can add too in terms of explicit foreshadowing which can pay off later in your story. In general, I view them as a nice literary device that you can take in a million directions.