I want to preface that this article is comparing traditional Whiteboard animation, not Whiteboard animation plus which integrates some of the strengths of animation and motion graphics.
If you are telling a story of characters doing things, animation is almost always the better tool (budgets not being a factor), a great example of this is a story in the template of the heroes journey. Pretty simple stuff, if the characters are interacting with each other or motion is required, than a still image will not do unless budget considerations mean an animated video is not an option. For this reason and others we have added quite a bit of animation to our videos where the still image is insufficient.
Whiteboard animation is the more useful tool when we are talking about the world of ideas, such as a new service, a process, an idea or a political issue. Whiteboard animation allows you to tell a non-linear visual story, presenting visual representations of the ideas and allowing the viewers mind to help weave the story. Of course motion graphics can do the same thing so why not make a motion graphic? Read here.