What are Dreams?
Some possible answers:
One purpose for the dream-state is as a kind of crash test laboratory in our minds. We can test out any scenario that we are likely to encounter in life, or a symbolic version of it, to see what we might do, and how the result will play out. We can repeat this as many times as necessary to find the course we will most likely take.
Where we can do, say and be anything we want to in a consequence-free context. Though we usually don't know it's a dream, our subconscious gets to play with all the component elements of our waking, conscious life: people, places, events. All of these focus our emotion.
Training for the Afterlife
Is there a connection between the dream state and that which we experience after the death of our physical bodies? Is the fluid physics-less world of dreams analogous to a body-less journey into the void that our souls will presumably embark upon? Dreams may be a kind of "practice" for the real thing, a gymnasium for us to stretch and build our spirit-bodies for what lies ahead. Or it may just be a reconnecting with where we were before birth.
Dreams are real...
They are very real in that they affect us. A dream can confer a feeling that stays with us the whole day, even a whole lifetime. They can empower or disempower us. These effects exist in the real world, and in this sense, the dreamworld invades into the waking one.
Dreams are inexplicable...
Can anyone fathom or explain how we all lie down and have the most amazing, wondrous, impossible and barely-intelligible experiences, and then wake up and go about our day as if nothing happened? What happens in dreams so often defies description. Things so strange that they cannot be put into words seem perfectly normal in the dream-state.
Dreams are important...
What would waking life be without the knowledge of this other, truly bizarre state of consciousness? We need some respite from a world where the laws of physics are unyielding, a place to relive, or pre-live any experience we like, from our lives or anyone else's. It is one place where we are unbounded, infinite, and able to connect with the divine. This must be a source of hope and comfort.