The line between something being an 'interesting/critical detail', and 'fluff/time wasting filler' is a fuzzy arbitrary decision best made on a case by case basis. As such we decide what to cut or what to expand based on what works for our story at hand.
There are two key metrics to consider when trying to decide if a section of text really belongs as is, needs to be rewritten, needs to be expanded, or should be cut entirely.
1. Is it interesting to read, and does it provide a satisfactory level of detail/content.
2. Does it fit well with the overarching piece? [Or, in the case where something ranks impressively high on the first metric, does the rest of the piece fit well with it...]
One of the best examples I've been given on why we should address the question of 'useful detail' vs 'needless fluff' with care is that we can carry things to the truly absurd:
"Character has problem. Character resolves problem. The end."
Reducing the majority of stories ever written to three highly questionable sentences shows us that we can obviously take various 'pruning' ideas much too far...
So the ultimate answer to the question boils down to what we are trying to achieve in a book with regards to overall look, feel, tone, etc.
This in turn raises a secondary question which needs to be answered before we can make much headway: What is the look, feel, tone, etc that we are after?
We answer this either by sitting down to carefully consider what we want to write, or by detailed review of what has already been written to try and pull and answer out of our existing work. But it is important to remember that writing is highly flexible. 487
What we set out to write is not always what we have by the end of things. Do not be scared to re-evaluate your work at various stages! [But do be careful of trapping yourself in unproductive loops while you waffle back and forth.]
Do you want your story to stay 'tight' and 'action packed', where everything keeps moving forward with a sort of excitable energy? Then your story will likely benefit from a 'less is more' for details in many cases, with careful consideration as to whether a specific point is worth adding/leaving in or not while you quickly move through your scenes.
On the other hand maybe you want a story that feels more like a long warm bath. Letting the reader slowly soak in a given scene gives us some rather different priorities where the writing decisions shift from 'can I safely gloss over this' and towards something more along the lines of 'how best can I phrase this detail'. Here ordering of details and 'setting the scene' can make or break the feel of a piece, and directly impact a reader's connection or view of a character. [Consider how a chapter on a character stuck in a waiting room may play out. What that character looks at or thinks, and in what order, impacts how the reader thinks about the character.]
As we gain a firm grasp of what we want overall in a piece of writing we gain a better ability to gauge how any given section helps with our goal: Does a section add to the overall work, does it drag things down, or would the overall work be improved by expanding on it?
Also keep in mind that not all readers belong in the same audience. Give careful consideration to your target reader, and be wary of trying to cast too broad of a net! Remember that even the highest ranked authors on best seller lists are still only reaching out to a relatively small chunk of overall readers. 487