According to the documentation, the PageSequenceMonitor key 'host' is used to set the Host: header in the HTTP request.
It seems that OpenNMS is using the 'host' value to override the nodelabel.
If the value cannot be resolved to an IP address, the request is not sent.
The documentation for virtual-host states that it is used to set the virtual host header.
In practice, it sets the Host: header.