All nightmares have purpose and meaning. They bring you in touch with a part of your situation you have been denying or have been unaware of. They assist in helping you make contact with something you have created. The nightmare is in one way a mask for the situation, and yet, in another way, it is an expression of the situation or problem. Just as people have nightmares while asleep, they can also have waking nightmares.
Nightmares are the only dreams some people remember.
This is a sign that their inner life is in turmoil. They are probably entertaining a belief framework that is at odds with their true nature. The nightmare, like most dreams, provides a kind of psychic therapy.