If there's a book you really want to read, but it hasn't been written yet, then you must write it. ~Toni Morrison
This is something I've always told my students - that even beyond writing for pleasure, or writing because it's something you "need" to do, it's also good to write a book that you, yourself, would want to pick up at a bookstore, purchase, and read. Something you haven't quite found already, from a book you've read. Something you need to write. For yourself.
In fact, that's partly the reason, I think, that I started writing my own series. I wanted to be transported to the Cotswolds in a book, and I hadn't yet found "that" book to read. So, I wrote one. I also wanted to write a story about a first love, an unrequited love, in a way I hadn't quite seen, either. Not that my plot is terribly unique, but I wanted to write a story like that from my own point of view. To see what I might do with a plot like that.
Also, I think that's the BEST mentality to have, regarding getting published. There's a fine line between writing something in order to get published (never a good idea), and writing something because we'd love to read it, ourselves, on our own pretend bookstore shelf. Then, if it ever gets published, wonderful! We wrote for ourselves first, and then someone else happened to read and enjoy it, too.