That isn't as easy of a question to answer as it might seem. Primarily because the answer has changed a lot over the years.
One thing that has remained a constant is that I enjoy to write. That was true when I started this blog back in 2009, and it's true now. I do think that a big driving factor for me starting the blog when I was sixteen was that it gave me space to write, and on top of that, there was a chance people could read it. Even if no one really did, that was exciting for me.
Back then, I wasn't aware of what book blogging was. I talked about books on my blog, but it was only because books were naturally such a large part of my life. I wasn't trying to write about any particular topic on the blog other than things I was interested in or doing.
Actually, I can't remember when I discovered book blogs. If I were judging based on my own blog, I'd hazard a guess that it was around late 2010 or early 2011 because that's when I can see my own posts changing to imitate what I know I had to be seeing on book blogs. It would take longer before I would go so far as to call this blog a book blog because, really, this blog was a mess of many different things. It still is truthfully, but it was even more so back then.
However, once I found that community my motivations for blogging shifted a bit. There was still the writing aspect, but now I also saw it as a way to write about (and, by extension, discuss) books, which was always something that I wanted to do. I discovered that the book blogging community was a great way to actually learn what was going on in the book world and that I could always find someone else who had read a book that I had. This blog became an outlet in another way.
From then on, I'm not sure that my motivations for blogging have changed all that much. Maybe they have in small ways that I don't notice, but the larger reasons are unchanged. I still enjoy having an outlet to write, and now I particularly enjoy writing about books (and other fandom things). Sure, I could do both of those things without this blog, but I think the blog intersects them both in a way that I might not get otherwise. I think that's the truly great thing about this blog.