If you are thinking of or are writing a story/novel series, that "and what next" question needs to be answered in depth for each of your characters to ensure you do have enough material to draw on to write about them convincingly for the next short story, the next novel etc. The world building needs to be able to withstand questions about it. People need to know how it works, there are no "gaps" to bring the whole "pretence down". I've found asking myself questions helps me flesh out what I want to write.
Writers, especially those aiming for publication, have to ask themselves this question regularly too. It is the way to review where you are now, where you would like to get to, how you are progressing and what you could do now to push that progress onwards.