Why do we have dreams?

like, is it preparing us for things that may happen in the future...or is it dwelling on things that have happened in the past?

That is really a complicated question, but I think we have them because of both the past and future.

its to process our experiances that day kind of like rapid file storage

I agree.
sometimes its sign that your stressed out in life and have a nonstop dream about the same thing over and over your brain is stuck on the stressful situation.
and sometimes your so chill and mellow that you have the best dreams ever. and sometime you don't dream at all or you can remember.
I dont think that answered you question but I tried
lol

Every thing in life can be answered with science. So think of it this way even during sleep your brain is still active, it's subconscious so lets say you didn't do your homework during sleep you will still be aware of this so you might have a bad dream of what might happen, or you have all A's and nothing wrong is happening so you have good dreams, it's all very simple.

it all depends
sometimes we dream to give our mind a creative outlet.

other times we dream to try and figure out our problems in our daily life, or we may dream something you feel but are to afraid to show it

dreams show our deepest thoughts and feelings and urges

My Psychology teacher said it's so we figure out the things in the day, that we couldn't figure out. Or things from the past.

It's usually the things that you think about before you sleep, or something you did during the day.

        Simply a Rose to brighten your day,
        And maybe lessen the cares in your way;
        And also, too, to help you to know,
        That in knowing you, many others grow!

 
Dreaming mostly results from your subconscious mind trying to process recent knowledge and experiences. It also attempts to deal with possible future outcomes as a result of possible experiences. The mind also freely associates different things some times when we are dreaming making for weird illogical dreams.

Most dreams aren't remembered. You have to train yourself to remember your dreams immediately upon awakening in order to get the best out of your dreams.