What is the goal of a blog, today?

Sorry, I've no answers for that, for now. I have my XWiki, my FaceBook, my Google+, my personal notes and productivity tools in the cloud, my papers and my books. I can write, record, annotate, read and share anything from anywhere.
Why, then, should I have a blog? What's the purpose of posting a blog entry, today?
I have a few ideas on the matter, we'll see whether they do work, actually.