Explain the concept of nuke from orbit.

Substantial difference between nuke from orbit and formatting disk is that malware may be also present on other components of your server than the disk. As this answer to the linked question already states, malware can by now - at least theoretically - affect any flashable, persistent storage. And since there are more and more components running a firmware inside one server, it would be really a PITA to verify each component's (flash) storage. E.g. someone could modify your NIC's firmware, creating backdoor access to booting the machine from a remote source (network boot) or enabling/exploiting any other remote management feature. Therefore: Nuke the whole machine!