In some cases you want to preserve the destination metadata, not the source metadata. For example, when updating an SSL certificate and key - they will often have special ownership to limit access (the key in particular). Rsync cannot be currently used without the user manually re-setting ownership.
If the destination file did not exist then rsync would fallback to its normal way to set metadata.
Apologies if this is already possible, I have not found a way to do this.