When writing change logs for version control, definitely include the "why".
When submitting patches, the reason for a change can tell the maintainers about your use case, which bugs it fixes, and also about potential regressions in other branches.
In five years, everyone can see what you did, but without an included reason, it will be lost and need to be guessed.
When hunting for bugs (e.g. via git-bisect), you'll want to know whether a commit can be reverted safely.
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