This follows from Freud's notion that dreams are mediating desires that are pushing for expression and attention and counter forces keeping those thoughts and impulses from disturbing the sleeper. Most of the content is disguised before reaching consciousness, but some gets kept away from the waking ego altogether. Hence the memory loss. Or at least, this is the theory. The clinicians feel that the notion is useful and the process easily observed over time.
New patients recall less dreams.
As patients begin to show other signs of less resistance to the material that the dream content is displaying, more of this dream content shows up. The higher the repression, the lower the content. However, these ideas are more anecdotal clinical observations than tested research. One approach to testing this is by using different personality measures. Yet the tests are mixed and not really convincing that it made any big difference what type of field-dependency you had, though there is a suggestion that field independent people recall dreams a little better.