From Nevill Coghill - The Poet Chaucer, 1949
"As for us [i.e. in 1949], so for Chaucer, dreams were a matter of scientific and philosophical enquiry, whether they were the dreams of poets or lovers or of humours of the blood, dreams of the soul or belly, dreams that were no more than a rag bag of past impressions and dreams that, being true visions of the future, had metaphysical importance since they seemed to establish that the future was in some way already fixed."
That preliminary 'As for us' shows the decline of the importance of dreams since 1949 - at the height of the influence of Freudian and Jungian analysis.
Tolkien's The Notion Club Papers (unfinished novel) is from this same era (1945-6) and has a similar fascination with the importance and possibilities of dreams.
And yet this recognition of the importance of dreams, which extends back as far as human records reach - back, even, into hunter gatherer times so far as we can tell - has now been displaced by the presumption that dreams are merely trivial. (I have even noticed a significant decline in interest in dreams during my adult lifetime.)
As usual, there are no discoveries to account for this displacement and downgrading of dreams - dreams have simply come to seem absurd, boring, irrelevant.
Dreams have not been explained - merely explained-away.
To recapitulate: (some) dreams used to be seen in spiritual terms as (potentially) providing of insights, supernatural guidance or power, perhaps prophetic, sometimes revelatory.
Then later on (and simultaneously, overlapping with the spiritual interpretation) dreams were seen as diagnostic - the medieval doctors would infer humoral imbalances from the nature of dreams, which had implications for treatment. Freud also saw dreams as diagnostic while Jung saw them as, in addition, either healing in their own right or providing clues to healing.
Now dreams are categorized as, at best, mere wish fulfillment - like a virtual reality home movie in the mind; or at worst nightmares that are unpleasant, hence require suppression or analgesia.
Dreams are yet another example of modern culture seeing-through, de-valuing, dis-enchanting and finally ignoring altogether a great swathe of human experience which had 'always' previously been regarded as highly significant and important.
Think dreams are useful? - Well, we know better!