One reason, mentioned, is in order to reserve premium features. Others are to maintain control over user experience; to simplify QA (making it less expensive,

One reason, mentioned, is in order to reserve premium features. Others are to maintain control over user experience; to simplify QA (making it less expensive, and leading to faster release cycles); reduce support costs; and increase engineering flexibility, by reducing the size of the API surface that requires a deprecation policy in order to avoid disrupting users.