But I don't understand why people want to block regular banner ads. Coming up with content then hosting it on a website isn't free.
Because of drive-by downloads.
Last year the ad network of a non-trivial Norwegian site was hacked, and they started serving malware which targeted Java. If the user hadn't updated Java fairly recently, they'd get infected without any user interaction.
The malware was designed specifically to target the largest bank in Norway. This bank required Java for their login procedure (they no longer do, took them long enough).
So, if the user visited this site with a vulnerable Java runtime, and then logged in to this bank later to pay some bills, the malware would send the money elsewhere.
Since the malware was running on the local machine, it could bypass the two-factor authentication (password+token) required when transferring money.