Have you not noticed that when you write something down with the purpose of summarising it in a structured way you tend to understand it better?

There's no market for kids essays, we never made them write these things with the purpose of being useful output. The market for essays for grown ups is also very small, making kids write things down was not with the purpose of covering the case of them becoming journalists or something. It's just that makes them learn to think and structure their thought process. That's why people take notes and write down things even if its not going to be graded or read by anyone else.