The blog is a great place to to do informal writing that you may or may not want to share with others. You can use your blog as a private journal that you don't publish at all.
Or you can work on entries privately and save them a drafts until you feel they are ready to share, and then publish them. When you decide to publish a blog, you can make it visible to friends only, to members only, or to everyone who visits the site.
If you wish, you can tag your entries with keywords so that others can find them more easily.
In short, the blog is what you make it.