According to to TechHomeBuidler.com in order to utilize the entire HomeKit feature set you will need an Apple TV. Some users are also experiencing trouble setting up voice control through Apple TV.
While you’re inside the home, Siri voice control works through the home Wi-Fi, but when you’re away, you need to have an Apple TV set top box to control your HomeKit devices.
Most of the devices released so far, including Lutron’s Caseta bridge, have their own smart phone apps, but that won’t allow homeowners to use voice control while their away.
You may have to spend some time with Apple tech support to get HomeKit to work properly.
Source: HomeKit Needs Apple TV for Total Smart Home Control – TecHome Builder