Often a story has to be left half told. Either you wrote too much and the book is too heavy on the hand and the mind. Or you had to stop abruptly because at that time you thought that would be a good idea.
Later, after you have written some more books and glancing through your collection of drafts you hit upon one that you feel could do with a bit more story.
We all know that feeling. It is like wanting your baby to grow bigger. But not unwieldy. Just the right size. In case your new addition leaves you wanting to say some more you always have the option of a serialised version.