Look at how language skills are measured. Tests do not only score reading and listening, they also evaluate writing and speaking. As many second language learners know, understanding a language is much easier than speaking a language.

The same applies to other fields and topics. Reading about something may be sufficient to have a rough idea of the concepts but you can't master a topic unless you externalize the knowledge. This means writing an essay, doing a project, teaching a course, or even just taking ACID notes.