Post meta registration became essential with the REST API’s introduction, which required distinguishing between public and private metadata. Without registration, WordPress keeps all post meta private to prevent accidental exposure of sensitive data.
The block editor can only modify metadata accessible through the REST API, making registration mandatory for any meta fields you want to edit through blocks. Setting show_in_rest to true makes fields available for block binding.
Beyond REST API access, registered meta provides a higher-level interface with built-in security, sanitization, and validation. Instead of raw get_post_meta() and update_post_meta() functions, you get managed data handling with default values and type safety.
Registration shifts you from low-level database operations requiring manual security and validation to WordPress-managed metadata with built-in protections and consistent behavior across the editing interface.