This reason is kind of lazy, but: Blogs help in making an awesome site devoted only to a particular subject and news about that subject easily. But the only criteria is: don't let it look like a normal blog.

For a personal blog: 1) Love writing about ideas and my thoughts even though a selected few would only read them. Makes me feel good and keeps my brain working.

2) After school, we didn't have a proper language subject, just computer science, maths ..., writing maintains the communication strength. Language is important too!