As for why Wang Xizhi's Preface to the Lanting Pavilion has traces of revision, this question is well explained. Just like we usually practice calligraphy, we occasionally make a few typos, what's more, a word may not look good. If you want to show it to others at this time, will it be revised repeatedly one by one?
Of course, we may throw away the written draft paper, but like this kind of literati, they will certainly save some useless handwriting, but there are also some handwriting worth collecting. Giving them up with the finished product may be more fulfilling.
Generally speaking, the so-called copying is to put the original in front of you, then imitate the handwriting and feeling of the original and write it yourself, which can ensure the coherence and fluency of the writing, but it will be very different from the original. Although the characters described do not have the characteristics of the writer himself, they are the closest to the original. However, double-hook copying is easy to paint ink, so the failure rate is high. The Preface to Lanting should have gone through at least hundreds of double hooks to get what it looks like today.