Some agents have so little information online that I feel like a stalker when I finally come across something. But that's not why agents should blog.
Some agents have so many clients, and are so good at their jobs, that they don't really need to be known. Those agents probably don't have to blog.
Some agents blog about stuff that has nothing to do with submissions, business, or publishing. That's totally cool, but that's not why agents should blog.
Some agents blog about writing and querying and publishing. That's extremely cool (I'm a much better writer for it), but that's not why agents should blog.
I figured it out while deciding which agents would go on my A-list and which on my B-list for querying. After taking everything into account -- genres they represent, deals they've made, stories they like -- I noticed a very strong trend: every agent on my A-list had a blog.
Now, probably, I'm just being a novice about this whole thing. My A-list agents should be ones making the big deals, or those selling stories similar to what I write, right? Then again, for someone who hasn't (and may never) go someplace where writers can meet agents, querying is very scary. And I don't mean the whole oh-my-gosh-I-hope-they-don't-reject-me kind of scary. I'm talking about the fact that I'm basically proposing a long term relationship with someone I've never met, I hardly know, and, let's be honest, whom I've been stalking.
Granted, that's how this business is, and there's nothing anyone can do about that. But when an agent blogs -- even if I only read a few posts before shooting off my query -- I feel like I know them a little better. I feel more comfortable. And in many cases, I feel certain I got their submission guidelines exactly the way they like it.
Blogging is branding (or if we're being technical, brand salience). You know how you'd rather buy Coca-Cola® than Generic-Brand Caramel-Colored Carbonated Sugar Water? It's not because Coke tastes better. Someone who's never had a soft drink in their life will be more likely to buy a Coke than GBCCCSW,* solely because they've heard of it. Because it's familiar.
I'd like to say I'm immune to branding. I'd like to say I choose my dream agents based on purely objective, business-minded decisions. But the truth is it's easier to ask someone out after you get to know them a little bit.
Even if you have to stalk them to do it.
* Also Jell-O instead of powdered bone slime.