What I write down is usually a quite literal dump of my brain. I have a problem, and rather than keeping it in my head, I write it down, and force myself to continue writing about the topic to force myself trying to find solutions, rather than be obsessed with the question.

Example: "I need to solve problem A. Problem A can be formulated in this way. This way is similar to a project I did a few years ago, if I remember correctly I had done B and C. However B would not work in the current situation, but would it not though? The issue is that it clashes with component X and Y. What about C? Hmm maybe but I needed approval from Z." etc. All of these thoughts are written down, without filter.

Forcing me to write down has two effects. The first one, slow down my thoughts, because discarding idea B after only 0.1 second of consideration is not productive if you do not explicitly think about why it is a bad idea, and consider the bad idea anyways. The second one is that writing down (especially manual writing and not keyboard typing, for reasons I cannot explain) allows you to think more deeply about your ideas, to envision it in different ways, not only the first way that popped to your mind. I think that keyboard writing requires too much of my brainpower compared to handwriting.

Moreover, in these sessions, having the possibility to look back to a previous idea immediately is extremely useful, and cannot be attained if you use an erasable surface rather than a notebook.

I have to say though that I very rarely look back to what I wrote after the session took place, unless I need to get back to the exact same problem.