One of the things that people don't like about smart home tech is how insecure they are. That's especially important if you're using a camera, because who wants some creep hacking in and watching you eat dinner? Well Apple's taking steps to secure that.
On stage at WWDC Apple revealed that lots of security cameras upload footage to the cloud to be analysed, but with HomeKit Secure Video cameras everything will be done locally. That means video analysis is done on another Apple device (iPad and Apple TV were mentioned), rather than by some internet algorithm. Footage is then encrypted and sent to your personal iCloud account.
Obviously you need a supported camera, which will probably be more expensive than most, but that's the price you pay for wanting to buy cheap.
Apple also revealed HomeKit will help keep you smart home more secure by interfacing with certain router. That way it can keep all your devices secure and firewalled, so should one be compromised the rest won't have to be. Obviously again you need a specific HomeKit-supported router, and probably won't work with your random Sky router.
Lot of privacy focus as expected, so don't expect this to be the last thing Apple has to say on the matter.