One of the most common forms of schoolwork is the writing assignment. The idea is that, if a student understands the subject, they should be able to demonstrate that by writing about the subject. These kinds of assignments are widely considered to improve comprehension and writing skill in addition to measuring comprehension.
However, there is a huge problem with this approach: writing assignments involve writing for an audience which already understands the concepts being discussed. The goal is not to communicate an idea, but rather to prove that the author already grasps that idea.
Needless to say, this is not at all the goal when writing outside of school. Outside of school, the goal is nearly always to communicate an idea the audience is new to.
One of the most important ways this distinction appears is verbosity. In school, using many, fancy words signals to the teacher that you understand the material. Students who expend the effort to understand the material well are also more likely to spend time writing many words.
But crucially, correlation does not necessarily imply causation: the willingness to write a thousand words on a topic is not caused by understanding of that topic, only correlated with it. The idea that someone must understand a topic well to write at length on it is entirely wrong. In fact, the ability to communicate an idea succinctly is one of the strongest marks of understanding.
Grading on word count entirely misses the point: it only encourages students to add meaningless padding. This makes students worse writers, and it doesn’t even measure understanding well.