What is the cause and purpose of nightmares as a mechanism of the normal human mind?

To some extent, it is the re-experiencing of past life dilemmas and giving them space to breathe and then allow a potential re-working which can bring healing value to bear. This will not happen if the nightmare is interrupted, or if a good solution is not possible for the subconscious to contemplate. This you know of as a sub-memory interference. The subconscious is not clever or astute enough to know what to do with this dilemma. So, when there has been a heavy trauma history many, many traumatic memories will come into awareness but the mind will not feel powerful enough to resolve them, and so, this keeps them on the front burner with no solution and that is the major difficulty here, not that the mind is incapable of doing the healing, it is just that when wounded too greatly it is not up to the task any longer, and then outside help is needed. This is rarely given because most people are unaware such techniques exist, nor the reason they are so valuable. So, the nightmares proceed, and are experienced unopposed and the person is a victim to the workings of their own mind reliving torment and projecting into new scenarios that are equally horrendous and will bring much additional emotional negativity into their energy overall. This adds to their problems and will create a state of anxiety, and this will have consequences, as well, during their waking hours as the subconscious will not necessarily let go of these and may return to them again and again, wondering about them, feeling frustrated, reliving the negative emotions involved in the trauma and projecting it again and again into future possibilities, and adding to the load of pain through the anticipated projections and then the anxiety will mount. And this is what can lead to states of fear and panic that often are inexplicable and that is because it is the subconscious level of the mind going through the torment, and the actual experiencing is not perceived by the conscious level of the mind, but the body itself will be in agony, as it is directly influenced by the deep subconscious. So, nightmares are a normal part of life when things go wrong.