Yes, such blogs tend to be very popular. How could they not be? The audience for such a blog is unpublished writers yearning for representation. Naturally enough, they leave comments on the agent’s blog that they hope will impress the agent. Then they constantly check back to see if the agent has responded.
The result? The agent’s blog gets many comments, and large traffic. It’s the same reason a butcher carrying a sack of bones into a kennel is very popular.
As to whether such blogs are helpful to the would-be author’s career? I dunno. Depends on the blogging agent, I guess. There’s not a lot of any substance for an agent to convey about what they do. For 15% of whatever money your book makes, your agent will email to their friends in the publishing industry the proposal for it (that you will have to write). That’s it. That’s what agents do: they email their friends in publishing.
And them doing that on your behalf is worth 15% of your book’s earnings, too. Whom do you know in the publishing industry? If the answer’s no one, your agent’s 15% is the cost of your admission into the that small coterie. And 85% of something is, after all, better than 100% of nothing. And nothing is what you’ll get if you try to approach consequential publishers on your own. Their clubhouse is not open to the public.
And it’s not the worst model in the world. Somebody needs to act like a filter for the big publishing houses.
Of course, those houses are increasingly desperate, and failing. The print that used to be king is being steadily usurped by the screen. The heavy, secret doors to the publishers’ clubhouse are deteriorating, crumbling, being blown away. But that’s really a whole other story.