It explains an observation I first made about 35 years ago, sitting on the rug and watching my family read.
The old assumption was that both eyes look at the same letter at the same time. Speaking as a reader, I had sometimes wondered about that, especially when you read in chunks rather than letter by letter. I felt like my eyes weren't quite together, and when I looked at my parents or brothers reading, especially when I looked from below, their eyes seemed to focus a whisker or two below the page. If they were absorbed and reading fast, their eyes seemed to focus further below the page, like they were physically climbing into the book eyes-first; if they were less intrigued, their focus was nearly exactly on the page.
It turns out that, when reading, the eyes focus a letter or two apart, and that polysyllabic portmanteau words can be read half by one eye and half by the other. The slightly split image is joined in the visual cortex of the brain by a process called fusion.
Fusion combines the slightly different images from each eye in the primary visual cortex, right at the rear of the brain. It serves several purposes: gives us the complexity of 3-dimensional vision; lets us interpret faint images more accurately; and, in reading, it apparently lets us move faster.
Focusing a letter or two, or even half a word, apart, would partly explain why my family's eyes seemed to be focusing slightly below the page. When they get around to studying avid readers reading avidly, I think they'll find that the eyes can diverge quite a bit more than a letter or three.