Suppose you have a report page template that you would like to apply to multiple Zabbix hosts. Instead of duplicating a page and updating host details in each and every graph / item on the page you can use a Host Selector macro to share server / host parameter values with other zabbix- macros.
Drop Host Selector macro on a page and specify the Server and Host parameters.
When you save the page the macro will print the selected Zabbix host name:
Now you can use Zabbix Graph or Zabbix Item macros without specifying a server or host: