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To learn more about the low level architecture of modern web frameworks in general.
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To create a framework tailored to your very specific needs (just be sure first that your needs are really specific);
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To experiment creating a framework for fun (in a learn-and-throw-away approach);
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To refactor an old/existing application that needs a good dose of recent web development best practices;
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To prove the world that you can actually create a framework on your own (... but with little effort).