It's the easiest way to serve an dynamic ad, it will require further configuration to get it to go through your own domain, and ad blockers block by element sel

It's the easiest way to serve an dynamic ad, it will require further configuration to get it to go through your own domain, and ad blockers block by element selectors and ad placement, so even those served through your domain will eventually get blocked. To avoid adblocker one would have to actually blend the ad with page moving it in the page to avoid getting detected, which might not be acceptable UX/UI.