Blog - what for?

At the moment, I am asking myself for what reasons people write blogs.

You cannot write everything on the Internet, you should be careful about what you write.
So, if a blog is kind of an online diary, and if you cannot write everything you would write into a diary, what sense does is make to write a blog?

And who would read your blog actually? Actually, above all friends should be interested in what is written on your blog. What if they do not want to read your posts?
Is that a sign of lack of friendship? Or rather a proof of friendship?
Does this friend show simply disinterest in your life? Or rather interest in your personal stories, the ones you tell only this person, and not the whole world?

To be honest, I cannot respond to those questions. But I have been asking them myself since a good friend of mine told me that he would not have time to become a member of my blog...