Something I wish I had started doing much earlier in my career was writing about the things I was learning and building. I wasn't always big into blogging or anything like that...that motivation didn't come until much later. But I've always been happy to teach others. And while it's very rewarding to help someone 1:1 or in small groups...it's not exactly scalable. I can only have so much impact if I have to be directly involved. For a long time now, I wish I would have written more down as I learned it, built it, or taught it. By writing it down, I would have been able to post it somewhere to share with others, or reuse when teaching, which is much, much more scalable.
Not only does writing down your thoughts and lessons allow you teach others and share those lessons, but it also helps you to solidify that knowledge in your brain. When you teach something, you end up learning that thing much more thoroughly. So even if you never share any of it with another soul, it's still wildly beneficial to take the time to think through what you learned, and write it down for yourself...heck, you may find it good for your own reference later!
When I'm talking to young developers, I always try to recommend they take the time to write about what they're learning and building. What problems did they run into? How did they solve it? What things stood out them? When I suggest this, one of the most common questions I get is, "How to do I know what's worth writing about?" It's a good question, and it took me some time to figure out exactly what is worth taking the time to write about? But finally, the answer came to me... "If it's worth knowing, it's worth writing about."
I personally encourage people to share the things they learn and build through writing, or some other medium, because if there's some piece of information they need, it's most assuredly a piece of information someone else needs. And it's only by sharing our knowledge as a large group are we able to build on one another's thoughts and ideas to advance the technologies around us. Think about where the world would be if people like Leonardo Divinci or Albert Einstein had not written down the things they learned and shared that knowledge with the world.
And if reinforcing the knowledge in your brain, or helping others isn't enough of a reason, let me give you one more. As you progress in your career, you'll be expected at some point to share some amount of knowledge with others. Whether that's because you're training some junior developer, doing a talk at a conference, or providing insights to your company's leaders, being able to share the things you know effectively is a skill all of us will need to possess at some point. So by taking time to write down that knowledge, you're actually practicing that very important skill.
So whether you're just trying to reinforce information in your brain, storing your knowledge in some external place for later reference, trying to help or teach others, or practicing for the day you have to share knowledge as part of your career responsibilities, writing down the things you learn is a very valuable thing to do. And I promise, you will thank yourself later. So what are you waiting for?!