[Swizec]

Illusion of explanatory depth.

Until you write (or otherwise explain), you really don’t know whether you even know what you think you do. We humans tend to over-estimate how well we understand something. We mentally paper over holes in our knowledge and handwave away pesky little details, until we try to explain the thing. Then you realize ”Wait, those two ideas aren’t connecting …”

https://en.wikipedia.org/wiki/Illusion_of_explanatory_depth

The other big reason, for me, is that without writing it soon feels like my head is exploding. So many ideas racing around it feels like I can’t think straight.

[nicbou]

Then you write code. I thought I understood how German health insurance works until I wrote a calculator for it. Suddenly I had to consider far more cases.

[zvmaz]

If I want to study something seriously, I formulate questions about what I am learning and commit them to Anki cards for review. It's anecdotal, but since I have been doing that, I am more careful of the content of what I study (I ask to myself: Did I understand that right? What does this part mean exactly? Isn't there a contradiction there? Let's see...).

[chrisweekly]

Yes. Taking it further, writing isn't just how you express your thinking; the writing IS the thinking.

[AnimalMuppet]

Feynman, in his freshman lectures, had some topic (I don't remember what) that he wanted to use as a topic. But he couldn't figure out how to explain it to freshmen. He said, "That means we don't really understand it."

[racktash]

It's generally good advice.

Although when I was younger I took it too much to heart and became obsessed with having a verbal and written understanding of everything when, sometimes, a deeper understanding at a subconscious, more intuitive, level is more useful. :)