Capturing the voice of a person is the gold standard for some ghostwriting.
Ghostwriting on behalf of narcissists, that is.
The alternative is to write for someone and make them sound the way they should sound - not just the way they do sound.
And the better copywriter will smile knowingly when that person says things like: "You've really captured my voice."
This is the end-game of good copywriting... and it's rare today.