Like any good English major, my first job after college was at my local library. I applied because I loved books, and wanted to be around them and people who loved them, too. But after the scent of paperbacks and sounds of turning pages faded, I noticed a vague sense of satisfaction at helping people find what they were looking for.
This was anything from helping a teenager locate a book, to showing a patron the best online resources for finding a new job, to directing a parent to the classes taking place this weekend.
Libraries are great not because of books (though that's a big part of it) but because it's a place of resource, where people can find what they're looking for and, sometimes, what they didn't even know they needed.
That's what UX Writing is all about.
If someone's looking for cookbooks, I can tell them to look in the Dewey Decimal section of 940, or I can say "upstairs and to the right." The answer I give will depend on the patron: to some, that code is meaningless, while for others they'll know exactly where to look. If a fifty-something pensioner is learning how to type on a keyboard, I can watch him clack away or direct him towards some practice videos.
Resources are useless if they're sitting undiscovered on a shelf. So when writing for the web, the content isn't the only thing that matters. How is the content organised, displayed, and catalogued? When is certain information presented to the user? How much info is shown, and when? These questions are answered by basic Information Architecture principles, like knowing where the user's coming from and where they're expecting to go.
Give the user what they need, when they need it. No more, no less. No one cares how the system functions; they just want to know what they can and can't do. No one wants to see every available option; they just want the one they're looking for.
Good writing, like a good librarian, is clear, relevant, and points the person in front of them in the right direction.