I'd say a consistent style or pattern is important, but illustrations, specifically, are not required. Check out the Harvard Law Review. It contains thousands of articles without an image.
Is that why so many medium posts have an irrelevant and distracting image that has to be scrolled past? What an idiotic requirement.
Medium claims to have awesome presentation but I always read their articles in “reader” mode to get rid of the pointless distractions.
https://www.nngroup.com/articles/photos-as-web-content/
> Summary: Users pay close attention to photos and other images that contain relevant information but ignore fluffy pictures used to "jazz up" web pages.
However, a lot of 'become popular on Medium' guides recommend you use an image, and an image likely looks better than a blank box does, hence all the images.