When it comes to learning, external representations, aka visualizations, have always been valuable tools to students. Visualizations play a key role in all learning.
So why are visuals important?
According to The Theory and Practice of Online Education: Students learn better from words and graphics or pictures than from words alone. There is the The Spatial contiguity principle, where students learn better when corresponding words and pictures are presented near rather than far from each other on the page or screen. The Temporal contiguity principle, where students learn better when corresponding words and pictures are presented simultaneously rather than successively. Lastly, The Redundancy principle, where people have only limited capacity to process visual and auditory material presented simultaneously; therefore, students learn better from graphics and narration than from a combination of other materials.
Overall, many theories support the importance of visualizations as a complement to learning. Incorporating visuals in online courses is both beneficial to the students, as well as the course, and has a profound impact on how students perceive information, learn, and judge usability. More articles regarding visuals in online courses can be found here and here.