Dreams

I often hear people say, "I didn't dream anything last night.".
But this is not the case - most just don't remember what it was that they dreamed about.

Inception was a decent movie - but it brought up some interesting ideas.
Why is it that most of the time we don't even know that we are dreaming?
Why do some of our dreams turn into nightmares?
I often know that I am dreaming and most of the time I just sit back and watch as though I was watching a movie with added emotions and sensations.

But what happens when we change and control our dreams?
Is the benefit such sleep any different?
Sometimes I edit and reset my dreams so that the results are always my desired outcomes.
As a result of thinking while dreaming - I'm never rested regardless of how many hours I'd slept.
Is the benefit of sleep and dreams derived from not thinking?
(From having 'faith' in the known-unknown?)