Daphne Merkin has a theory as to why we care about them:
The intimate lives of writers have always had a special attraction for readers, perhaps because we imagine that people who can shape ideas and arrange scenes on the page should be able to offer us some special insight into how to order our messy off-the-page lives. This has rarely been proven the casewriters often seem less, rather than more, gifted at the mechanics of everyday existence; all the same it has not stemmed our interest in finding out what Sylvia said to Ted or why Simone pimped for Jean-Paul. This interest speaks, I think, to a dream of coherencea matching-up of intellect and emotion, of romance and reasonthat continues to inspire us even as it eludes our grasp.
Of course some writers read literary biography for other reasons. Nick Mamatas confesses:
I get most excited when reading about rejection slips and advances and late royalty checks and underfunded publishers with the can't-fail idea to print a lot of quickly written short books and those novels written in a feverish six weeks to pay off a tax bill and the shocking disappointment that comes with only selling a few thousand copies of a novel that ends up widely read only posthumously. Since my own career is mostly indie press stuff that doesn't sell at all, I don't even bother to mentally adjust for inflation. Did Nathanael West only make $780 in royalties? Gee, me too. And if Lovecraft got a penny and a half per word out of Weird Tales, I managed five cents a word from that same magazine only 77 years later.