Ooooo, this is a hard question!
Other times, I come up with an interesting character for the story I'm writing, and once they're on the page they become way more fascinating than I thought they would be. That happened with Isobel's sisters in Rage. Once I started to write about the different personalities of the sisters, and the dynamic within the family, I knew I wanted to tell their stories too. Hence, the upcoming Sinclair Sister Trilogy.
Sometimes, I know who will have their own future book before I write them as a secondary character into someone else's book. This was the case in the first three Invertary books. I knew before writing Lingerie Wars that the heroine's best friend would get her own book at some point. And she did, in Goody Two Shoes. I also knew that the Goth shop assistant in Lingerie Wars would get her own book, and she did too, in Magenta Mine.
I guess the main reason for giving someone their own book, is that they become interesting enough for me to want to write their story. Of course, this can work the opposite way too. There have been a couple of characters I was sure would have their own books later on down the road, but once they were on the page, I lost interest in them.
And sometimes, you plan for a character to be part of the secondary plot in your book and then wish later on that they'd had a whole book to themselves. This happened to me with Grunt and Claire, in Calamity Jena. Calamity Jena is obviously about Jena and Matt, but the secondary romance (all of the Invertary books have a secondary romance!) was with Matt's sister Claire and the American bodyguard Grunt. As soon as Grunt and Claire met I knew they deserved a book of their own, but I couldn't rip them out of the story I'd put them in to. To this day, I regret not finding a way to give them their own book. Who knows? Maybe one day I'll revisit them and do a book about how they're getting on now. If nothing else, it would be fun to see how the taciturn, man-mountain, Grunt gets on as a father.
Hope that answers your question Chessela!