Why Vox makes its own banner ads

Vox says that because its ads are built on its Chorus CMS, advertisers can better target their custom ads to readers based off their interests, which the company says increases click-through compared to regular banners.
With standard display units, a brand doesn’t have to create a new ad specific to every publisher it advertises with. Custom units, in contrast, take more creative overhead and are limited to the small handful of sites that run them. Quartz said that 65 percent of its advertisers ask it to design and engineer their custom units. Five to 10 percent build their own ads from start to finish, using preexisting assets. The process is relatively high-touch compared to the automated ad serving world.
 http://digiday.com/publishers/vox-quartz-custom/