To control a light, some hardware has to exist somewhere (in the bulb/socket, or attached to the light) to control it. Hubs centralize the control of multiple lights so that apps can interface with one device, along with some identifying address, to turn the light on/off/dim. App talks to hub, hub to light, action occurs.
Homekit extends that concept somewhat allowing things like discovery of existing devices in your home. Apple TV may allow Siri to turn lights on/off by voice. The ATV will be a hub of sorts then, and will likely be interfacing with the vendor hub for actual actions. In this case it might be along the lines of: Talk to Siri, ATV recognizes command, ATV talks to vendor hub, vendor hub to device, action occurs.