Nagios Core only comes with a barebone structure for plaintext-based e-mail alerting. These add-on scripts take the notification further by adding HTML formatting, color coding, language selections, and more. Integration with two popular graphing packages Nagiosgraph and PNP4Nagios adds problem visualization to the alerts.
This shortens the time for problem identification, increases problem transparency, and decreases time-to-fix.
See also:
http://nagios.fm4dd.com/howto/nagios-flexible-notifications.htm
http://nagios.fm4dd.com/howto/manual/nagios_send_host_mail.htm
http://nagios.fm4dd.com/howto/manual/nagios_send_service_mail.htm
http://nagios.fm4dd.com/howto/manual/pnp4n_send_service_mail.htm