Some quests, particularly those from Fortitude: by the Docks of Big Lake, connect to normal worldbuilding, referring to people or situations you can read about in the prose, but that's not what I'm really interested in here. I'm more talking about stuff like "you use an elaborate, magical-looking key". Why is there an elaborate key? What is it used for? That's for the player or the HG to figure out, but being on the quest The Trials of the Sun gives support for such a key to exist for whatever reason. A lot of the weirder ones give a cop-out option; e.g. "You meet a star in human form" has a footnote saying that "a glamorous enough celebrity is also OK", and some suggest that the relevant events might happen in a dream sequence, so in some sense they're less facts and more prompts. I find a lot of them very evocative, and like a good writing prompt I feel like they could push you in directions you wouldn't think of otherwise, serving as an evocative seed to get you to define your world more interestingly, rather then telling you exactly how something fits into the world.
Like XP Actions, storyline quests can also have some of the "this is what you should spend time on" nature. XP actions say things like "you should have slice of life scenes" in a Pastoral game. Some quest flavor is like this, but more specific; if you're on a Beautiful and Far Away quest, you should be doing things like "talking with somebody about what dreams are, as compared to reality". That's a game feel sort of thing, and pushes roleplay in interesting directions like some quest actions push worldbuilding.
I think both of these end up pushing the HG and players towards symbols and metaphor in an interesting way. Quest flavor can include prompting you to watch birds flying, or to eat hot cross buns or melon pan, without telling you what that means or why that's relevant to something like reinventing yourself. Leaving the significance of such things open-ended seems like it could lead to cool play in a way that'd be hard to do in normal sourcebook prose. And it keeps the feel of the game and the author a continuous, changing part of play, rather than something that's involved in character creation but falls by the wayside after that.
Basically, storyline quests seem super-evocative in a bunch of different ways, and I feel like it provides support for the feel of a game in the way standard sourcebook text can't, while also changing over time to keep things fresh. I look forward to seeing more of them in my new Chuubo's campaign.