Each dream, including nightmares contains images from our day-to-day life. Sigmund Freud states that each thing we dream has a symbolic meaning which lead to the fulfilment of our subconscious wishes.

During REM sleeping, the neocortex reviews the neural connections and dumbs the unnecessary ones.

Dreams involving dangerous situations are very common and the Primitive Instinct Rehearsal Theory holds that the content of a dream is significant to its purpose.

• Stress neurotransmitters in the brain are much less active during the REM stage of sleep even during dreams of traumatic experiences.
• This made some scientists to think that one purpose of dreaming is to take the edge off painful experiences to allow for psychological healing.
...

Not being limited to the rules of conventional logic, the mind can create infinite scenarios to help you find solutions that you wouldn't think conscious.

To increase the performance on some certain tasks, sleep is good, but dreaming while sleeping is better.