By reading weblogs, you can learn stuff. So can you by reading other articles online, that’s not particularly surprising. What is special about weblogs is that you can also learn by writing articles.
Unless you are writing a stupid copycat article, writing an article requires quite a mental effort. To write a good article, you obviously have to understand what you are talking about. It requires a deeper understanding, because you have to transform your knowledge so that others can understand what you are talking about as well.
Generally, writing a blog article forces you to think a lot about your topic. Many might know this phenomenon from university. You often learn more when you have to write a paper about a topic than when you simply attain to the lectures.
So next time you want to learn something, why don’t you write an article about it? :)