@sbundi are you trying to install the Nagios integration extension? This one is a separately licensed extension developer by Alanata (Dynatrace Partner). Please reach out to firstname.lastname@example.org for a trial license if you are interested.
You can find more about the extension here: https://alanata.atlassian.net/wiki/spaces/DTNAGIOS/overview
@larry_roberts Reasons for the extension vary, however typically customers do have some investment in their Nagios installation and want to use that data - such as custom integrations or hardware monitoring. It's also not uncommon for customers to have limited Dynatrace licenses (budget) and cannot cover the whole environment with OneAgents.