How does reading for pleasure do its magic of enabling children and school students to access education - as all research evidence on the matter shows?
There are some answers that I wouldn't argue with but are a bit limited in their view, like: 'reading for pleasure widens readers' vocabulary' or, 'it enables readers to empathise with others' and so on. Just to be clear, I don't disagree with these but I think we should look further in the matter:
1. The way we write is different from the way we speak in many ways. Reading for pleasure is the simplest and most enjoyable way to get the structures and methods of the many ways of writing. This is at the level of 'vocabulary' but also at the level of 'grammar' or 'how the language is stuck together in sentences and paragraphs', and also at the level of how ideas are structured and carried through in fiction, non-fiction, drama, poetry, graphic novels and the like. We can 'teach' these things as much as we like but unless we are exposed to a lot of it, we won't really 'get' it in ways we can easily access it and, in turn, write like that ourselves.
2. Fiction is a special form of writing which combines ideas and feeling attached to beings ('characters') who we care about. This matter of it being about both ideas and feelings at the same time, creates a great potential for speculation about matters of principle, fairness and a good deal of thought about appropriate or inappropriate ways of going on. This is a generator of powerful and important thought, which in turn overlaps with a good deal of what we talk about in the context of education in schools.
3. As we read, we interpret. This is a process to do with weighing up possibilities, probabilities, figuring out meanings, coming up with feasible conclusions. It's a key part of our means of survival. The bizarre 'truth' put about by the test-crazy regime is that what is important about reading is 'retrieval and inference'. These are just means to the greater end of 'interpretation' - the really important thing we do to understand what we're reading. In an ideal world, classrooms would be full of the sounds of children and students sharing interpretations - indeed as some (many?) are. Reading is potentially a gateway into developing our powers of interpretation.
4. Following this, in an ideal world we can discover that interpretation is not a matter of coming up with these strange exam-led formulae about guessing what the examiner thinks the writer thought when he or she wrote a given metaphor, or used alliteration. We never know for certain why the writer did this or that and most certainly, there isn't one perfect, 'ideal' reader who is affected by the metaphor or alliteration in the way that the examiner thinks this ideal reader is affected. This narrows the potential of writing to enable and foster interpretation. To get to that point we should go with the flow of writing, and explore how we think and feel - and why. Finding the links to other things we've read and our own life experience and back to the writing in question is one way of doing that.
5. Through free browsing and choosing we discover how to scan writing for our own benefit and use. We develop our autonomy and agency and discover what kinds of writing 'work' for us. In so doing, we get more and more confident to surf, scan, 'plunder' texts for what we need and want. This requires exposure to piles of books, shelves of books, many texts - wherever and however they can be found. Libraries are ideal for this, of course.