There are several novels I’ve had to quit working on because they just weren’t working out for me. I no longer had the incentive to revise them or make them better because I felt my time would be better spent on other projects. Some might argue that it was a waste of time, effort, and energy to take a novel through three or four revisions, then just give up on it.
For me, it wasn’t a waste of time at all. I got some writing experience under my belt and I have a better sense of what works and doesn’t work in stories. Also, I have some awesome characters that can be given a new lease on life in a different novel.
There are some characters I’ve grown very attached to over the course of writing novels I’ve given up on, so why not reuse them in other stories, even if it’s just a small cameo appearance? And since many of the novels I’ve written take place in the same small town, it only makes sense that certain characters would show up repeatedly throughout a series or throughout a sequence of related books.
Including characters from previous works also reduces the amount of time you have to spend coming up with even more characters to remember and develop. It saves energy, basically.
So the moral of the story is… no novel is ever wasted, even if it never sees the light of day. Take the good parts of it and use them in other projects. Put your characters in new situations and see how they react. They may turn out to be a lot better in a new story than in the one you had them in.