It's basically a medieval to-do list and/or a text-based scrapbook.

• Journals
• Travel logs
• Edgy hypercommercialized Keri Smith books

• Supposed to be a dump of whatever loose concept is on your mind
• A method to catalog and document things you've found
• Not meant to be elaborate, though not necessarily exclusively disheveled

• Read and learn enough. You'll eventually find an urge to take a load off your mind and put it down somewhere.

This is in fact something that happens and everybody does it to at least some minimal extent. You start with a list of things to do, then a list of stuff to remember, then eventually one wants to keep information from somewhere else and writes it down. Eventually one derives this behaviour into a hobby that is merely a more accepted but still compulsive variation of (information) hoarding. An “External Brain” or however you wanna call it; it is a human tendency so well-grounded that it has prevailed in separate cultures: all societies¹⁾, isolated or not, have had inhabitants who kept commonplace books in some sort or another.