When you have characters but no plot
You know exactly who you want on the page and nothing about what happens to them. Type the characters and one thing about them, like a rivalry, a debt or a secret. The generator treats that as the seed of a plot and asks what would make it move: what forces them together, what keeps them apart, and what one of them is hiding from the other.
Ideas that hold up for a whole fic
A good idea has a question the fic has to answer, and the follow-up questions are built to find it. By the time you pick the heat and the length, you have a premise with a source of tension, a setting and a point of view. The first chapter is written from it in the full editor after a free sign-up, and you can steer every chapter after that.