There's not yet a lot of information about this feature. In short:
With HomeKit, it's possible that apps can lock and unlock doors, turn on and off lights, open garage doors, turn on and off IP cameras, ...
Apps that use HomeKit to control devices for home automation and requested access to home data, have to be enabled in your Privacy settings. As it's for such things as unlocking doors as well, it might be clear why you find it there.