I (like most of you, I'm sure) spent my formative years in the pre-Internet era as a voracious reader. I idolized my favorite writers, but, for the most part, didn't know anything about them beyond what was written on the book jackets. It seemed, for the most part, that these stories I loved were created by distant, unfathomable people who I could never hope to understand. As I grew, I began to realize the foolishness of this, but as I struggled with my own writing and continued to fail to meet anyone with any sort of literary fame, the idea that authors were somehow superhuman stuck with me.
When I began using the Internet and discovered that writers had their own sites and (eventually) blogs, and they interacted directly with their fans on a daily basis, it blew away my naïve childhood preconceptions: these were regular people who I could talk to. It also helped demystify the professional writing business, and relieve me of any romantic notions that still lingered about solitary geniuses pecking away at their typewriters and sending glowing, polished manuscripts off to publishing houses.
I don't feel it's necessary for writers to blog; probably more than 95% of what I read is written by writers whose blogs I don't read, if they even have blogs, and I don't feel like I enjoy their stories any less. But when a writer does blog, it feels more like they're carrying on the old oral storytelling traditions: a piece of fiction isn't a discrete work that exists in a vacuum; there's a real person telling it, and that person is approachable.
I also don't think that the comfort and familiarity that reading an author's blog lends to their fictional work necessarily gives them a leg up on other writers; I will just as gladly read a story or a novel written by someone who doesn't blog as someone who does, if the story is an award winner or recommended by friends, for instance. I don't base my buying decisions on whether I read a writer's blog, but I do tend to read the blogs of authors whose books I choose to buy.
Lastly, I don't expect authors to blog if they don't want to. I don't need that familiarity to enjoy a work, but it does feel like it adds something to the experience. Not something essential, just a little bit of extra perspective.