(with the usual proviso that this is probably something to be worked out on meta once we enter private beta)
Learning about authors helps us to appreciate their works better. If we want to learn more about works of literature (or any art, really), part of that involves looking at the people who created them. Why did they write it like this instead of like that - what inspired them to do so?
- Part of the reason why Orwell's 1984 ended as depressingly as it did was because while writing it, Orwell was suffering from an illness which eventually killed him.
- Many of Edgar Allan Poe's stories and poems concern the death of a young woman close to the narrator - these were inspired by the early death of Poe's own wife.
Knowing these details about the author's lives helps us to analyse their works.
Besides, where do we draw the line? Let's look at the example in your question, as well as some of the highest-voted example questions from the proposal itself:
How has experience in WWI influenced Tolkien's writing?
This is actually more about Tolkien's books than about the man himself. The question is deliberately phrased in terms of the influence on his works, which should place it very firmly on-topic despite involving details of his life outside of writing.
When and why did the Brothers Grimm start collecting fairy tales?
The highest-voted question from the definition stage, this could be seen as more about the authors than their works. OK, it's about how their magnum opus came to be, but any answer would likely be about their real lives and not about literature per se.
Where did Margery Allingham talk about "polishing her prose until it shone overbright"?
This again seems to be more about an author than her works as such.
One thing we might learn from the above examples, especially the first one, is that questions about authors are more likely to be on-topic if phrased in terms of their works. For example, "Where exactly did Tolkien serve in WWI?" might or might not be considered on-topic (we'd have to find out in practice how such a question would be treated), but "How did his WWI experience influence Tolkien's writing?" surely would be.