A blog frees memory space in your mind; once you’ve written an idea down you don’t need to worry about forgetting it.
Blogging is basically storytelling which has always been important for communicating information and entertaining. These days, telling a story well is important for forming connections and influencing people. Your ability to tell a story is sharpened every time you write.
A blog functions as a note to the future. Down the road, you can look back at a snapshot of what was happening and what you were thinking by rereading a blog post.
Creating a blog teaches you a new skill, constructing a website.
A blog gives you a unique online presence. If your blog is good, you can probably use it in place of a resume, showing more of who you are and what you can do. Just attach a cover letter.
Writing makes you organize your thoughts, clarifying ideas before they turn into the swamp gas of your marshy memory.
Old friends who discover your blog seem to enjoy catching up with you.
Publishing on a schedule builds discipline.
A blog can help you make sense of an externally chaotic world, like a rock in a river. Blogging allows you to share your musings with a potentially large audience, and maybe even build up a tribe.
There’s also a factor of serendipity, you won’t really know where your blog will lead. I saw this site’s ranking, it’s at position 6,658,374 out of about 450,000,000 (active blogs in English), not too bad. If you enjoy writing and sharing, that’s good enough.