Apple’s HomeKit platform uses iCloud Keychain to securely store your home data through end-to-end encryption, using the highest level of security. Your iCloud Keychain information uses a unique key that is specific to your mobile device and your mobile devices passcode. Your home data includes the unique product key, product names, home name, rooms within the home, other HomeKit products, groups, scenes, other users, and automations.
This allows the home data to securely sync from your primary iOS device to a secondary iOS device such as an iPad, Syncing Between iOS Devices. The HomeKit platform uses iCloud Keychain to securely sync the home data over to additional users you choose to share your home with, Share Control of your Accessories on iOS.
To have access to all the features of iDevices products and the Connected app, it’s best to turn on iCloud Keychain. https://support.apple.com/en-us/HT204085