A recent discussion on Hacker News asked, in effect, what makes a good blog, which in turn asks the question, “why write a blog?” There’s no perfect answer; as Scott Rosenberg’s Say Everything: How Blogging Began, What It’s Becoming, and Why It Matters indicates, people write for practically as many reasons as there are people: prestige, boredom, ego, whatever.
That being said, the best blogs focus on specific niches but often use those niches to explore the wider world. For example, Marginal Revolution is nominally an economics blog, but it also discusses foreign travel, ethnic cuisine, books, and more. The blogs I contribute to try to follow the same general principle: the one you’re reading now focuses on books (this focus can be very broad: some of my posts about keyboard reviews, for example, get a lot of traffic) and Grant Writing Confidential discusses grant writing. The latter in particular has a purpose beyond random musings: it’s there to show people how to write proposals and that we know how.
If you’re thinking about writing a blog, read Penelope Trunk’s comments, which are invaluable if not always accurate. In addition, I wrote a post called “You’re Not Going to be a Professional Blogger, Regardless of What the Wall Street Journal Tells You” that got slashdotted and ought to dissuade you from the idea you’re going to make money directly at it, at least in the short term. But if you’re looking for a means of expression and you want to write primarily because you want to write, then just roll with it.
“Without a purpose for writing, though, I don’t see how to even try writing a blog. Any suggestions?”
As others have said, don’t write a blog if you don’t have a purpose. Your purpose should come from something you care about deeply enough to know something about that you’d like to transmit to others: in my case, that means books, chiefly, but also grant writing. For many HN readers, it probably means programming. Remember too that the deep knowledge/writing/transmission process isn’t linear, but recursive: I’ve probably learned more about books by trying to sort my ideas about them out in a logical, rational way than I would if I just read a lot (this, incidentally, is why good schools require you to write a lot: writing forces you to embellish the ideas you do have and often to come up with ideas you didn’t have previously). Sometimes you don’t need a purpose in advance: you’ll find one as you start writing.
It’s been said by various writers and scholars (see, for example, Ian Watt’s The Rise of the Novel or Michael McKeon’s The Origins of the English Novel) that the novel is the genre that consumes all other genres—that is to say, it can contain elements of epic poems, Romance, poetry, history, philosophy, and more. By the same token, blogging is the genre that can subsume any other genre if you want it to, because blogging is more a form than a way of presenting content, and over the past 10 years we’ve hardly touched on what is possible.
Just don’t write about your cat. That’s the only rule. There are enough blogs about people’s cats.
See also Scott Rosenberg’s Say Everything: How Blogging Began, What It’s Becoming, and Why It Matters.