Serving advertisements is certainly very bandwidth and energy intensive at least when talking about news/media sites.
The majority of the data that is getting pushed around during a page load is ad delivery or tracking code to inform the ad delivery. That is a lot of servers, load balancers etc etc working hard pushing around ad code. The actual text of a news article is a tiny part of the whole workload.
Once the site is being loaded by your browser (ie you have downloaded all the JS and your machine has parsed a few MB of ad/tracking code), depending on the site's ad provider(s) there is potentially a real time auction going on for every ad slot on every page load. Multiple ad networks hold automated auctions within themselves which are rolled up into an auction between the winner of each of those auctions. There are literally banks of big beefy servers scattered around the world bidding on ad slots 24/7.
A breath-taking amount of energy and bandwidth get used up to display an annoying 'sweater for dogs' ad next to a news article.
They do significantly more than this, even if you wanted to (incorrectly) classify all of their products like this, they actually do more than that to entice people in.