What is the importance of dreams in A Midsummer Night's Dream? How do they affect the outcome?

To me, dreams are important in the play, but I do not think they actually affect the outcome.  Instead, I would say that the dreams emphasize the meaning of play.

The whole play is meant to be something of a fantasy.  There are fairies and love potions and mistaken identities and stuff like that.  Because of that, the characters are often unsure if they are awake or dreaming.  The dreaminess of the plot line, in my opinion, emphasizes how unreal our lives can feel -- how strange and out of our control they can be.

But the dreams don't really affect the outcome.  When Hermia dreams of the snake, for example, it doesn't really change what she does next.