In Fallen London, there are Opportunity cards where you can experience dreams, which usually give you an increase in Nightmares, which can be inconvenient and/or expensive to deal with. The Opportunity cards that give you dreams are relatively few, relatively far between, and increase your Dreams qualities very, very slowly.
I ask: aside from exploring the odd content written for dreams, what are dreams good for? Do they unlock future storylets other than the progression to other dream cards with the same drawbacks? Will I eventually be rewarded with new and interesting content, improved connections, or possibly cold, hard cash? I am hoping to find some sort of reassurance that the Dreams in Fallen London either have a point, so I have a goal to wait for, or don't have a point, so I can stop following them.
(On a personal note, I find prose about dreams boring: it's disconnected, nonsensical, and never really goes anywhere. Even in stories that feature some sort of prophetic or mental-description-of-the-dreamer sorts of dreams, the link to the waking world is tenuous at best.)
Best Answer
As you've gathered, dreams are chiefly long, atmospheric storylines that provide a small amount of orthogonal content. While you don't need to complete dreams to complete the story, the dreams do give you insight into some of the themes and main storylines of Fallen London.
However:
Completing What The Thunder Said gives you the Stormy-Eyed quality, which does turn up outside dreams.
You can trade progress in Dream stories to Dr Schlomo and, in certain circumstances, the Manager of the Royal Bethlehem.
There are a handful of options in the Cave of the Nadir, that place of dreams and forgetting, that are unlocked by dreams qualities.
Dream progress can be hazardous at high Nightmares, as additional Nightmare-inducing cards will appear in your deck.
Death by Water at a specific level also used to be important for beginning the Seeking Mr Eaten's Name storyline, but no longer. Unfortunately, it is now much easier to set yourself on that path, in the Forgotten Quarter.