> I have a site that gets about 30k pageviews per month; I threw ads on it just to see what would happen. It makes something like $12/month

This seemed surprisingly low to me, so I did a little Googling.

https://blog.adstage.io/google-display-ads-cpm-cpc-ctr-bench...

> In Q1 2018, [AdWords] advertisers paid $2.8 dollars per thousand impressions

So for a site with 30K monthly page-views, I'd expect something closer to $84 per month with one ad per page. Place an ad on both the top and bottom of each page, and now it's $168 per month. Still not a ton by any stretch, but an extra 2K in income each year is nothing to sneeze at.

Ad rates are continually dropping, so I'm sure the current figure is lower. And you're assuming a random blogger would get the same kind of yield as the mostly professional participants in the ad network there.

And even if you're right, an extra $2k in income would only be nothing to sneeze at if the ROI is decent. But competition for eyeballs is fierce. There's a reason that even professional writers are having a hard time making ends meet.

... and it makes a world of difference for some people.

Ad clearinghouses are a flattening and democratizing force on the Internet that I don't think people recognize as such. They let people who otherwise couldn't really afford to pay for server space put up their content.