You get your opinion out to the world.
You find other people from all over the world that like the same books or authors that you do.
Lets you get your feelings out if you're traumatized by some books and their endings.
Lets you have a sense of community with other book bloggers out there.
Sometimes feeling like nobody reads what you write.
Easy to get sucked in by some of the drama out there.
Sometimes forget to have fun and not make the blog feel like work.