Fiction, like the real world, is supposed to be messy. It is, however, a continuum: if fiction were as chaotic, random and often pointless as reality, readers would revolt. On the other hand, a story where all the pieces fit neatly into place and everything has a purpose feels too contrived, too convenient. It breaks our suspension of disbelief.
I have a pet peeve with prequel stories, mainly illustrated by film examples. For me, I find myself pulled out of the story when the prequel tries to explain / incorporate everything. Isn't it awfully convenient that the complexities of the now-time story can be contained within this one episode? To me, rather than illuminating mysteries and revealing new facets, it makes the fictional world feel smaller.
(My personal examples: White Collar and the prequel episode that finally shows Neal's relationship with Kate; the Spartacus prequel series.)
A related issue is the amount of content in a story that isn't strictly necessary. Once again, I feel moved to offer a culinary metaphor. I hope y'all will forgive me.
We're told to cut fat off our stories - but fat is what provides flavor. Lean pieces of meat are frequently wrapped in or threaded with fat while cooking to ensure taste. When hunting for a quality piece of meat, we look for marbling. And what does marbling represent? That, my friends, is intramuscular fat. In the fictional world, I suppose that would be "unnecessary" detail that fits so smoothly in with the story the writer is telling that it never stands out.
This may be why I've never had much luck with shorter stories. I do luck upon them occasionally: She's Unable To Lunch Today, which I just finished, runs about 2800 words ... but even that tale has some detail that doesn't pertain directly to the story and characters at hand. I just find it difficult to narrow my focus to only the essential elements. There's so much just around the corner, and it only takes a heartbeat to take a peek ...
By contrast, this has served me well in Unnatural Causes. It makes for a great method of generating red herrings.
I feel the urge to extend the culinary metaphor once again, but I hate fish.