Zabbix provides features to cover all the items that cannot be retrieved with the standard agent. In real life, it is possible that you are not able to install the standard agent on the device that you would like to monitor. A practical example is the UPS, all the servers that, for some reason, cannot be compromised when installing external software, or all the appliances that cannot have custom software installed.

Now, for all those reasons, you cannot have an agent on your device but you need to monitor the vital parameters of this device, the only feasible solution for which is to use an external check.