It's not just the void: it's the fact the arrow points directly to the link in the top banner ad.
It's also almost perfect for the audience: it's aimed at cartoonists where intentionally crude stylings are usually intentional.
(I doubt it would be similarly effective if promoting SaaS or luxury travel, where it would look jarringly out of place)
It also has a much higher proportion of screen real estate than the average banner campaign and apparently little of visual interest to compete with it, both of which are critical to its success.
Ugly ads are sort of known to do well. This is why those "one weird trick" ads do so well. Also the weird sketches of people done in ms paint and junk. It's about getting attention and clicks. And it works.
Yeah, this makes sense to me in retrospect. As a user of AdBlock[0], I forgot all about those ads. What inspired me was a cheap banner ad for Louis CK's comedy album[1].
I've installed adblock before because of ugly ads: specifically the "weird trick to get rid of belly fat" with a horrible drawing of... belly fat. (shudder)
A small bit of encouragement - a site I like switched to Disqus for commenting some time ago. Specifically, Disqus with included advertisements. They always seemed to be the same ones, including one with disgusting imagery (I don't recall exactly what, but probably similar to your belly fat ad). Eventually I noticed the link to file a complaint with Disqus... and I never saw that ad on the site again.
The text on the sidebar was also quite clever in referencing itself.
To me, it mostly shows that clever and clear copy wins over nice graphics.