Writing small summaries weekly (or daily) about what’s on your mind or what you’ve learnt is the simplest and most powerful way of self development I know of.
Do you review what you've written at a later date or is the value gained just in the initial writing?
I read it, yearly, but the main value for me is probably identifying what I need to do/understand in order to “get to the next step” of whatever I’m currently trying to do.