A device registry answers a basic question for every incoming connection: do we know this device, and is it allowed in? Each record ties a unique identifier to a credential such as an X.509 certificate or a symmetric key, plus metadata like model, firmware version and owner. Platforms consult it during authentication and use its attributes to target commands and updates.
Keeping the registry in step with reality is the hard part. Devices are built, shipped, returned, resold and scrapped, and each step should be reflected or you end up with orphan identities that can still connect. Automated provisioning at manufacturing time and a clear decommissioning procedure prevent most of this drift.
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