Have you ever read a book, and six months later, when you recall it, you can't remember much about it? Try doing it right now. Seriously—take a second and think about the last book you read. How much do you remember from it? Now think about a book you read one or two years ago. If it was fiction, can you recall its major characters? If it was non-fiction, can you remember all of the significant arguments?
Remembering in detail is challenging.
I've found that writing about the things I've learned helps solidify my memory. I've always learned best by doing. Passively watching programming tutorials and mimicking the instructor doesn't do it for me. A deeper understanding happens when I take off the training wheels and start programming on my own. I have to play with a technology to master it.
I like to think of the web as a massive mosaic of knowledge. To me, that's its most significant innovation: the open sourcing of information. This is what enabled me to become a programmer. All the information I needed to learn programming was freely available on the web. Think about how incredible that is. Anyone in the world with a computer, an internet connection, and enough time can do the same.
So why do I write? Because I want to deepen my understanding and contribute to this worldwide web of knowledge.