Host actions page can become real handy if you are using a virtual HyperVisor (ESXi, Xen etc), or your server has a management port (IPMI, iLO etc). You do not have to set it, as it only applies to hypervisors physical containers or management port.
If there is an issue with a virtual hypervisor that runs 15 virtual machines, there is no need to send 15 alerts. Instead, we will first check if the hypervisor is alive, and then we will continue to check it’s hosted machines. If the hypervisor is dead, we will send only one alert about it, and all monitoring actions for the hosted virtual machines will be suspended until it comes back alive again.