It is a source of issues when /var/lib/tuleap/ftp/tuleap is not a symlink (e.g. when data have been retrieved from a Docker based install) and you try to upgrade the Tuleap package.
It causes issues like:
error: unpacking of archive failed on file /var/lib/tuleap/ftp/tuleap;66156359: cpio: File from package already exists as a directory in system