This hook is used in the body Accessor of a page. Every time you'll call $page->body this hook will be triggered. You can still use ->getOriginal('body') on the PageTranslation entity like any laravel model to bypass this accessor.
getBody(): Will return the current body content
setBody(''): Sets the body element
getOriginal(): Get the original body without any changes by other listeners
You could use this hook to add short codes to your page body section for example.