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As I recall, Lord Byron, Mary Shelley, Percy Shelley, Horace Smith, and whoever else their friends were, used to challenge each other to write things. Quite a few novels and poems came out of those...
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#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/43895 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/43895 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
As I recall, Lord Byron, Mary Shelley, Percy Shelley, Horace Smith, and whoever else their friends were, used to challenge each other to write things. Quite a few novels and poems came out of those friendly contests. A writing challenge forces you to step out of your comfort zone, stretch your creativity. Your "serious project" is a rut. Sure, as you go forwards, you learn, your skill grows. But it's small exercises like the ones you talk about that let you step out of the rut, go sideways in all kinds of directions, exercise your creativity muscles. How is that helpful, if your "serious project" is your main goal? Imagine you're working on this serious dark tale. And then, for an exercise, you have to write something humorous. You come back to your dark tale having put some skillpoints in humour, and suddenly you see how you can add a touch of humour to some moments in your serious tale. You can bring light into the story, so the shadows would look even darker. Or, you've been writing stories on all kinds of prompts as exercises, and now your main project got stuck. With the experience you've gained creating stories from prompts, you can now use those same skills to get your main story unstuck. A particular short story might not help your novel very much. But the continuous exercise of writing new and varied things should help you become a better writer, improving both your current novel, and the following ones.