Not strictly true. There is a difference between a copy of a Bach concerto and the original score in Bach's own handwriting, even if the copy captures all the notes perfectly. Original manuscripts can contain scratched-out parts which are nevertheless of great interest to scholars (for example, Kafka experts are deeply interested in every scratched-out word from Kafka's originals). Handwriting can convey information about whether the author was writing in a hurry, or had the luxury of taking her time. One could even fantasize about discovering hidden messages encoded somehow in the original handwriting, which would be totally lost in a naive word-for-word copy.
"See what large letters I am using to write to you with my own hand!" (Galatians 6:11)