Why are plugins needed?

This is a basic question. If Vera3 Lite is compatible with all certified z-wave devices, why is a plugin needed for some of the devices. Thanks

Plugins allow non Z-Wave devices to be included in your home automation system, such as legacy alarm panels, TCP/IP-based thermostat and weather stations, serial-controlled home energy monitors, etc., etc. Virtually any device with an electronic pulse can be integrated into Vera.

Which Z-Wave devices specifically are you referring to?