What are dreams for?

I question the purpose of having dreams.

I have no issues if I have happy dreams whereby I wake up feeling happier and good. I don't exactly have dreams every night 'coz on some days, I am dead tired and fell asleep within minutes. However, there are days whereby I am talking or doing stuff up till the minute I sleep and then, somehow my dream will be an extension of the reality. Its quite irritating 'coz I wake up feeling unrested.

Then, there are those unexplained dreams that I keep having.

I need to google for answers.