Because otherwise, multiple passes of chef-client were needed to properly converge, because of wrong ordering on the first run:
1. Install Apache
2. Delete /etc/apache2/conf.d/nagios3.conf
3. Install Nagios (creates /etc/apache2/conf.d/nagios3.conf)
4. Reload Apache because of new configuration, load conf.d/nagios3.conf

Maybe it could have been resolved with an immediate notification instead.