WHY MUST YOU ALWAYS MAKE ME LAUGH????????
Dreams are, like what Livewire said, projections of the subconscious, fantasies, etcetera. Imo, dreams always have a meaning or a purpose, random or not. Even if it has nothing to do with you, it still has a reason on why you're dreaming about it. The reason depends on what you're dreaming about -- for example, you dreamed about winning the lotto. It's one of those fantasy wishes you're wishing about, and therefore, it pops up in your head while you sleep.
Does that make sense?