For added security, it would be desired for some users to store the tmp/ folder (currently: the path/to/piwik/tmp/ folder), into a custom path such as
/tmp/piwik (outside of the website itself so that it's not accessible from internet).
another reason why this might be a good idea is that the debian package places the
tmp directory inside
/usr/share/piwik, which afaik should not contain variable data.