If we never remembered any dreams, then dream study would be a fairly
difficult enterprise. Nevertheless, the default behaviour is to forget
dreams. Usually dreams are remembered only if we wake up when they occur.
The dreams that we do remember are only a small proportion of all the dreams
we have. Some people hardly remember any dreams at all, unless deliberate
efforts are made to wake them during the occurrence of REM sleep.
If it is important that dreams be forgotten, then this is probably because the actual dream content consists of false information, that is, the dream situations did not actually occur. After all, if we remembered all our dreams, we would often confuse dream with reality in our memories, leaving us with a somewhat inaccurate knowledge of our own personal history (i.e. even more inaccurate than it would be otherwise).