Maybe you hope for a modicum of fame. Maybe you hope to make a bit of money from it. Surely writing is still one of the ways you an earn a living? Maybe you want to hang out with your favorite authors as colleagues instead of just fans.
Maybe you want to be remembered after you've fled this mortal coil. Maybe you just have a story in your head that's driving you batty and you need to get it out of there.
Whatever your reason, it can be helpful to think about what it is you want from your writing. It's hard to judge whether you've been successful if you don't know what success means for you.
Tell us in the comments what you hope to get from writing a book.