Plugins are actually the core of the system. You can only communicate with hypervisors and virtual machines through the plugins. Each hypervisor has it’s own plugin, inwich you can setup hosts, and import virtal machines from the physical host.
There are 3 basic plugin actions – reboot, turn on, turn off. This gives you and your clients ease of access for controling the server through WHMC’s admin / client area.
VMM can also perform automatic action based on WHMCS hooks. For example, you can automatically suspend a machine when the product was not paid for.
Although it’s not a virtual machine, but plugins are also useful for controling IPMI based servers (iLO for example) in case you will need to reboot the entire physical server.