The act of externalizing your thoughts—even vocally—requires clarifying and completing your ideas, which, similarly to writing, leads to better understanding. What's more, since when you're discussing something you're talking to others (not to yourself like when writing), you must tailor the information to the recipient. Usually, this entails simplifying the information, which often forces you decompose it into its building blocks (PN: The first principles), which usually exposes how much you understand it.