Not for anyone else, it's how you notice how much has actually changed, and what you'd genuinely do differently if you were starting over today.
A year of decisions is hard to see clearly from inside it. Writing down the specific advice you'd give your day-one self forces you to name what actually shifted, instead of leaving it as a vague sense that you've "grown."
Write it as if you're actually handing it to someone
Specific enough to be useful, not just "trust the process." What would you actually tell them to do differently in week one?
Include the thing you got wrong that you'd fix earlier
Everyone has at least one. Naming it honestly is more useful than a highlight reel of what went well.
Revisit it at year two
What you'd add or cross out a year from now is its own kind of progress marker.