Sanitization is secondary function. Main is:
The register_setting and unregister_setting functions add and remove options from a whitelist of allowed options that the form is able to save. They can also name a sanitize callback function as a security measure to check each option’s value. ( Register settings )
Technically $option_group argument is used for:
- array key in global array of whitelisted options
- part of filter name to run sanitization callback